PDF file size, page size and zoom: three different sizes

Do you mean storage space, paper format or view?

With PDFs, the word “size” stands for three properties. File size counts bytes and determines whether a file fits under an upload limit or into an email attachment. Page size is the page format stored in the PDF, measured in millimeters, such as A4. How large a page appears on paper also depends on the chosen paper and the print scaling. Zoom in percent only describes how large your viewer is currently showing the page on screen.

These values aren't fixed in relation to each other. An A4 page with plain text can take up a few kilobytes, while an A5 page with a large photo can take up several megabytes. Here's how to classify your task:

• A portal reports “File too large” or names a limit in MB: It's about file size.
• You want to print an A4 document on A5 as a booklet, flyer or notepad: It's about page size.
• The page doesn't fit in the window or looks too large on screen: It's only about zoom.

File size: reducing KB and MB

With a size limit, what counts is the exact byte count of the saved file. The unit matters: 1 MB is 1,000,000 bytes in decimal, 1 MiB is 1,048,576 bytes in binary. Not every recipient says which base they mean. With the calculator, you can tell KB, MB and MiB apart and estimate tight limits cautiously.

To reduce the bytes, Reduce PDF file size in MB changes the encoding and structure of the file. The lossless method optimizes structure and data streams; the other two methods can re-encode suitable embedded images as JPEG. The page format stays unchanged: an A4 page is still an A4 page afterwards. How much can be saved depends on the content of the PDF.

An optional target size counts as reached if the result is less than or equal to the chosen byte value. If a requirement explicitly asks for “under 2 MB”, you therefore choose a target size just below it. If no reduction succeeds, you receive an unchanged copy. At the end, compare the size of the downloaded file with the recipient's exact limit.

Page size: printing A4 on A5

In ideal dimensions, A5 has exactly half the area of A4. If you cut an A4 sheet in half across the long edge, you get two pieces of 210 × 148.5 mm, so roughly A5 using the rounded standard dimensions. Nothing is shrunk in the process; the sheet is just separated into two parts. If, on the other hand, the whole A4 page should fit on an A5 sheet, its content is scaled proportionally, and this leads to the most important decisions when printing.

Dimensions of A4 and A5

From A4 to A5, width and height don't shrink to half, but to about 70.7 percent. Only the area is halved. With the rounded standard dimensions, slightly different values result (148 ÷ 210 ≈ 70.5 %, 210 ÷ 297 ≈ 70.7 %), and your printer's margins further reduce the printable area. That's why there is no percentage value that fits exactly on every printer. You can also find the dimensions in the Adobe overview of the A5 format.

Does A5 make the PDF smaller?

Not necessarily in bytes. Whether a page measures 210 × 297 mm or 148 × 210 mm says little about how much storage the text, fonts and images on it take up. If you print an A4 PDF on A5 paper, the file stays unchanged anyway. Print settings don't rewrite the stored page box, in a PDF usually the MediaBox, and a photo remains stored with the same pixels.

Print an A4 PDF on A5 in Acrobat

The Acrobat help on page size when printing describes this method for Acrobat on the desktop:

1. Open the A4 PDF in Acrobat and choose “Print this file”.
2. Choose your printer and set A5 as the paper size. Whether A5 is available depends on the printer and its page settings.
3. Under “Page Sizing & Handling”, choose the “Size” area and there “Fit” or “Shrink oversized pages”. This scales the A4 page to the chosen paper.
4. If “Choose paper source by PDF page size” is enabled and the printer therefore doesn't use your A5 paper, disable this option.
5. Check the orientation, margins and small text in the preview before you print.

“Actual size” doesn't scale. An A4 page will then be cut off on A5 paper. Under “Custom scale”, you enter a percentage value yourself and check margins and orientation more carefully accordingly. An older Adobe article on scaling printed pages opens the print dialog on Windows via the menu with the three lines and on macOS via File > Print. The labels may differ depending on the version.

These settings apply to the printout; your A4 PDF remains an A4 PDF. If you choose a PDF printer as the destination instead of a paper printer, a separate new file is created. Whether it has A5 pages, which properties of the original are preserved and how large it is in bytes depends on the PDF printer and its settings. So open the saved file and check the page format, content and file size.

Fit, fill, crop or split?

Behind “A4 to A5” are different operations with different results. Decide beforehand what should happen to the content:

Because A4 and A5 have almost the same aspect ratio, proportional fitting mainly leaves the printer margins visible. Much wider white strips appear, for example, when portrait and landscape orientation don't match.

Zoom and pixel dimensions: only the display changes

If you zoom out to 50 % in the PDF viewer, the page is drawn smaller, but the file stays the same. A 2 MB PDF still has 2 MB and the same number of pages at 50 % zoom. A viewer such as PDF.js calculates a different display area in pixels from the same page depending on the scale, as the PDF.js examples show. It doesn't change the bytes or page box of the file. For a limit like “under 2 MB”, you therefore need real compression and then a look at the bytes of the saved file.

Why the same page has different pixel dimensions

When an entire PDF page is rasterized, for example on screen, when exporting as an image or when printing, a new pixel representation of that page is created. How many pixels it has depends on the chosen resolution. The effective resolution in PPI is the number of pixels divided by the length in inches, and one inch measures 25.4 mm. Text and vector graphics in a PDF don't have a fixed resolution for the whole file. Embedded photos or scans, on the other hand, are already stored as pixels and bring their own pixel dimensions.

The calculator you can use to calculate print size and pixels rounds to the nearest whole pixel count, like the middle column. If a requirement demands at least a certain resolution, round up yourself as in the last column. Which resolution is sufficient is determined by the recipient's requirements and your check of the result. When you export pages as JPG or PNG, a new raster image of the page is created. This doesn't change the page format of the PDF.

Choose the right next step

Once you know which of the three sizes you want to change, the path is clear:

• The byte limit is exceeded: Reduce PDF file size in MB and compare the saved file with the exact limit.
• The A4 PDF is to be printed on A5 paper: scale it in the Acrobat print dialog with “Fit” or “Shrink oversized pages”. The original PDF stays A4.
• You need print dimensions, pixels or PPI: Calculate print size and pixels.
• You want to create a new PDF from photos or scans: Create a PDF from images with a chosen paper format. The options are A4, Letter and the original size of the image; there is no A5 preset there.
• You only want to keep certain pages: With Split PDF, you select whole pages or page ranges. A scanned two-page spread is not split in half.

With the tools on this website, you cannot permanently change the page format of an existing PDF to A5, crop margins or split two-page spreads in half. For that, you need a program with its own function for page format or cropping. Afterwards, check in the saved file whether the content, margins and page count are correct, and only compress it after that if a byte limit also applies.