Compress a PDF for email: telling attachment size and message size apart

Which limit applies: attachments or the whole message?

Email limits don't all measure the same thing. Some limit the total of the files you attach, as they are stored on your device. Others limit the complete message. That includes the attachments in their encoded form, your text, inline images and technical headers. In addition, your own mailbox and the recipient's mail server can each have their own limits. Your email has to stay within both.

Gmail: 25 MB for all attachments combined

According to Gmail Help: Send attachments with your Gmail message, personal accounts have a limit of 25 MB for all attachments combined. That's not 25 MB per file. Two PDFs of 15 MB each are therefore already over it, even though each one on its own is well below. If the limit is exceeded, Gmail automatically replaces the attachments with a Google Drive link. In work or school accounts with Google Workspace, the administrator sets the limits. There, your organization's rules apply. The help page doesn't say how many bytes Google counts as one MB. If your total is very close to 25 MB, leave yourself some margin.

Why Outlook reports different figures than Gmail

“Outlook” isn't a single mailbox with one fixed limit. Microsoft states different values depending on the product, and these values measure different things. For Outlook.com, according to Microsoft Support on Outlook.com, there's a limit of 25 MB for the entire email, including text and inline files. For the classic Outlook program with internet accounts, Microsoft Learn: Outlook error for attachments that are too large describes a default of 20 MB for attachments in total. For Exchange accounts, the limit configured on the server is what counts. So Gmail and Outlook don't contradict each other; they refer to different accounts and different reference quantities. Even the same figure of 25 MB doesn't mean the same thing for Gmail and Outlook.com. If you use a work or school account, it's best to ask your IT department which limit applies to your mailbox.

How to find the figure you compare against:

1. Determine which account you're sending from: personal or through work or school, in the browser or in an email program.
2. Check whether the limit applies to a single file, all attachments together or the whole message.
3. Check whether the recipient sets requirements of their own, for example in a job posting, a letter or an error message about an earlier email.
4. Note down each limit together with what it refers to. Your message has to meet all of them at the same time.

Why an attachment appears larger in the email

Attaching the PDF on your device doesn't make it larger. For sending, though, it's rewritten as text characters, because email transport is designed for text. The usual method for this is Base64, described in IETF RFC 2045: Base64 Content-Transfer-Encoding. Every 3 bytes of the file become 4 characters. An incomplete final group is padded to 4 characters with padding characters. The encoded lines may be at most 76 characters long, and each line break costs 2 additional bytes. Around the attachment come the message headers, your text and the MIME boundaries that separate the individual parts.

Base64 is purely a re-encoding, not encryption. When valid Base64 is decoded correctly, it yields exactly the same bytes as your PDF. There's no fixed percentage surcharge that's right for every email, because text and headers vary from message to message. But you can calculate exactly the portion that comes directly from the attachment.

Why does 4.9 MB become more than 6 MB in the outbox?

This very effect can explain why a file shows about 4.9 MB but the email in the outbox shows more than 6 MB. In the hypothetical example above, an assumed 4,900,000 bytes already produce an encoded attachment of 6,705,266 bytes, or roughly 6.7 MB, even without text and headers. The PDF itself hasn't changed.

With a real message, two points remain open. First, it depends on the program whether “4.9 MB” means exactly 4,900,000 bytes or was calculated using 1,048,576 bytes per MiB. You can work out the difference under Check file size units. Second, only the limit that actually applies decides whether 6.7 MB is too much.

Compress the PDF and calculate the message

Depending on the limit from the first step, you'll end up comparing either the bytes of the saved file or the bytes of the encoded message. The process is the same in both cases: first you compress the real file, then you calculate with its actual bytes.

1. In the compressor under Compress a PDF before sending, use “Choose PDF” to select the original PDF, not a copy you've already compressed. Files up to 50 MiB (52,428,800 bytes) are supported. Processing runs in your browser; the file isn't uploaded to a processing server for this. Your original stays unchanged.
2. Start with the “Balanced” method. If that isn't enough, try “Smaller file”. This can recompress embedded images more strongly. “Lossless optimization” only changes the internal structure without deliberately changing image quality, and in return often saves less. If no method produces a saving, you get the original back.
3. If you have a fixed figure, open Compress a PDF to a target size and select your original PDF again there via “Choose PDF”, because the file and result from the compressor aren't carried over. The limit of 50 MiB per file applies here too. Then enter your target size; the result shows “Target reached” or “Target not reached”. Pages aren't cut off to achieve this. If your limit applies to the whole message, set the target well below the limit, because the encoding is added on top.
4. Compare “Original” and “Result” page by page with “Previous comparison page” and “Next comparison page”. Then save the result. Small print, tables, stamps and signatures should remain easy to read.
5. Enter the exact byte count of the saved file and of every other attachment individually under Calculate attachment and message size. You'll find the byte count in your device's file properties.
6. If you want to check against a limit, first tick “Compare with a size limit” there. Then enter the limit with its unit and choose the matching reference: for a limit like the one for personal Gmail, use “Total of original attachments”; for a limit like the one for Outlook.com, use “Estimated encoded message”. The preset value of 25 MB is only a starting example; replace it with your real limit. For text and headers, you can enter an assumed allowance in bytes. The default is 0; if you don't want to add an allowance, leave the 0 in place instead of clearing the field.

Always enter multiple attachments individually rather than adding up their sizes beforehand. Each file is encoded separately and gets its own padding characters and line breaks. If you calculate the total as a single file, the result can therefore differ slightly.

The compressor doesn't accept digitally signed or certified PDFs or password-encrypted PDFs. A scanned, visible signature isn't a digital signature in itself.

So the same file can fit comfortably under one limit and be clearly too large under the other. That's why you shouldn't simply compress to a round figure like 10 MB. Clarify what the limit refers to first, then the figure.

Can I use the calculator to guarantee that the email arrives?

No. The calculator works with the sizes you enter and the allowance you assume yourself. It doesn't build a complete email, doesn't send anything and doesn't know the recipient's mail server. A passed comparison only means that your figure doesn't exceed the selected limit; a value exactly at the limit counts as fitting in the calculator. If the recipient asks for “under 5 MB”, choose a slightly lower figure. If you're checking against a limit for the whole message and are very close, a margin you choose yourself gives you more room for text and headers you don't know exactly. For a limit that only covers the original attachments, you don't need a buffer for this.

When a link is the better fit

Sometimes a PDF can't be compressed far enough without becoming unreadable. Or several documents together exceed the limit. Then a link to the file can be the better route, provided the recipient accepts links. Some recipients, for example for job applications, explicitly require an attachment. In that case, follow their requirements.

When Gmail sends a Drive link

In a personal Gmail account this happens automatically: if the attachments together exceed 25 MB, Gmail inserts a Google Drive link instead of the files. For the recipient to open the PDF, they need access to it. According to Google: Send Google Drive attachments in Gmail, Gmail warns you if a recipient lacks permission. Which sharing options are available can also depend on your organization's rules. Share a personal or confidential PDF only with the people who are meant to receive it, rather than opening it to anyone with the link. A delivered link also doesn't mean the recipient accepts cloud links. Some employers don't allow them.

This website doesn't store files for you and doesn't send emails. You create the link through your email or cloud provider.

Other options if the PDF stays too large

• Regenerate instead of squeezing further: If the PDF comes from a word processor, a fresh export can produce a smaller file. For scans, a new scan at a suitable resolution helps; more on this under Compress a scanned PDF.
• Split it if several emails are allowed: With Split PDF you distribute the pages across several files. How large each part becomes depends on its pages.
• ZIP only with care: A PDF that's already well compressed often gets barely smaller in a ZIP archive, and some recipients don't accept ZIP files.
• A PDF stays a PDF: If the recipient expects a PDF, don't send the pages as JPG images.

If the file doesn't get smaller at all, or only slightly, when compressing, you'll find the typical causes under PDF won't get smaller.

Check before sending

Before you click Send, it's worth doing one last pass with exactly the file you're attaching.

• Open the saved PDF once more: are all pages there, in the right order and readable throughout?
• Attach exactly the checked file that meets your limit and is easy to read. That can also be the unchanged original if it already meets the limit.
• Count everything: all attachments and, for a message limit, also text, email signature and inline images.
• For an attachment limit like that of personal Gmail, you compare the saved bytes of all attachments together. An encoding surcharge doesn't belong here.
• For a limit on the whole message, you compare the encoded attachments plus text, headers and MIME boundaries.
• Also meet the recipient's requirements, even if your own mailbox allows more.

If the email bounces

Read the error message carefully. If it states a size and says whether the limit applies to attachments or the whole message, recalculate using exactly that information. Also check which files were actually attached. If it remains unclear what the recipient allows, ask for a route they accept, for example a link or several emails.