Compressing PDFs with signatures and forms: what may be changed?

Signature image or digital signature?

In a PDF, the word signature can mean three different things. A signature that is drawn, typed, scanned or inserted as an image is, first of all, a visible mark. Depending on the program that created it, it is stored as text, as a graphic or image, or as an annotation. You usually can't tell which from the outside. It also doesn't matter when you are asking about a digital signature, because the mark itself is not a certificate-based signature that your PDF program could verify. Many signing tools create marks like this, for example the fill-and-sign feature that Adobe describes (Adobe: Fill and sign documents).

A digital signature, by contrast, is created with a digital ID. A digital ID consists of a private signing key and a certificate that others can use to verify the signature. The signature is a feature of the file, not of the page image. Adobe distinguishes between approval signatures and certifications. A certification specifies which later changes are permitted, and it can be visible or completely invisible (Adobe: Digital signatures overview).

The third state is an empty signature field. This is a form field intended for a later digital signature that doesn't contain one yet. As long as nobody has signed in it, the field contains nothing that could be valid or invalid. However, this applies only to the field itself. If the file contains a completed signature or an invisible certification elsewhere, it is still signed, and our tools will reject it.

Appearance alone therefore proves nothing. A signature image can look almost identical to a visible digital signature, and a certification can be present in the file with no visible mark at all. Your PDF program shows whether a digital signature is present and valid. Open the signature list and read the validation status, the certificate properties and, if available, the timestamp (Adobe: Validate digital signatures).

A scanned signature is therefore not a digital signature, even if it sits in the right place at the bottom of the document. It shows how the document was signed. It does not prove who signed it, and it does not prove that the content has stayed unchanged since then.

Why our tool rejects signed files

A digital signature covers precisely defined byte ranges of the file. During compression, objects and streams are reordered and rewritten. This process therefore cannot guarantee that the signature will still match the bytes that were originally signed. The compressor does not rewrite a signed PDF anyway. Instead, it examines the file structure before processing and rejects the file as soon as it finds signature characteristics.

So will your digital signature remain valid after compression? No such assurance is given here, which is exactly why a signed file is not compressed at all. Some programs allow certain changes after signing, for example when a certification explicitly permits them. A complete rewrite of the file, which is what compression does, is not something this process can safeguard.

What leads to rejection

• a completed signature in a signature field, even if the field inherits its type from a parent field
• signature objects and signed byte ranges, even if optional markers are missing from the file
• certifications and signatures for extended usage rights that govern later changes
• encrypted or password-protected PDFs
• files whose structure already triggers warnings when they are read

A rejection says nothing about whether your signature is valid, trustworthy or legally sufficient. The tool does not check certificates or their revocation status. It only detects that the file contains signature characteristics. Your selected file is not modified or overwritten in the process, and there is deliberately no option to bypass the check. For the other limits that apply and how the tools handle your files, see Check document limits before processing.

A quick check before you try again

• Open the original file in your PDF program and look in the signature list to see whether a signature or certification is listed there.
• If a signature is listed, the rejection is expected. To continue, use an unsigned version, as described in the last section.
• If the file is encrypted, you need an unprotected version from whoever created it. The tool does not unlock files.
• If you know the document is signed and the tool still doesn't reject it, don't process it. XML signatures in XFA forms are not explicitly among the characteristics checked.

Forms: the compressor and the page tools have different limits

A fillable form consists of more than what you see on the page. Each field has stored information such as a name, a value and an appearance, and often also formatting rules, calculations or scripts. Classic PDF forms are called AcroForm. XFA is a separate, XML-based form format.

The compressor optimizes objects and streams and keeps pages and fields in the document. This means ordinary form fields, links and tags can be preserved. A reduced-size version is only used if it passes an automatic structure check and has the same number of pages, form fields and links as the original. In addition, the reduced version must have a tag structure if and only if the original has one.

The page tools work differently. Merge, Split, Delete pages and Rotate copy pages into a new PDF file. Exporting to JPG or PNG instead renders the pages as images, which contain only pixels and no editable fields or other document features. Form fields, however, belong to the whole document rather than to a single page. As a precaution, all of these tools therefore reject any PDF with interactive form fields or XFA, even if the only field is an empty signature field.

This is why the compressor can accept a form that is rejected when you try to split it. The tools perform different tasks and therefore have different rules. The same applies to a PDF with an empty signature field. It may be eligible for size reduction if nothing else prevents it, but not for page operations such as splitting or merging.

The automatic comparison in the compressor counts fields and links. It does not run them. Matching counts do not mean that calculations, formatting rules, field actions, scripts or link targets still work unchanged. There is no dedicated functional check for XFA forms and scripts. The next section explains what to check yourself after saving.

Note that the preview may display your file before a tool rejects it. A successful preview therefore does not mean that the operation you chose will accept the file.

Flattening is not invisible preservation

Flattening, which German-language programs often label "Reduzieren" or "Flatten", copies the visible appearance of fields and annotations into the page content (QPDF documentation). The fields can no longer be edited afterward. A page that only looks filled in is no longer an interactive form. Whether it looks exactly the same as before depends on the program, its settings and the form, so check this in the saved file. Our tools do not flatten fields automatically. Some workflows combine flattening with compression. In that case, flattening is an additional conversion that deliberately changes how the file works. It is not an invisible side effect of reducing its size.

Appearance is not enough to check functionality

A page preview draws what a page looks like. It does not show whether fields still calculate, whether a link still has its destination, whether the reading order is correct for screen readers, or whether a signature is valid. The automatic structure check also has limits. It examines the syntax and structure of the file, but its documentation states that it does not guarantee that all content fully conforms to the PDF specification (QPDF documentation). In addition, the presence of tags does not by itself demonstrate a meaningful reading order.

So does the preview prove PDF/A or PDF/UA conformance? No. Neither the preview nor the automatic comparison confirms that a file meets an archiving or accessibility standard. If conformance is required, check the saved file with a validator built specifically for that purpose.

How to check the saved copy

1. Save the result as a new file and keep the original.
2. Open the saved file in the program that you or your recipient will use with it, not just in this website's preview.
3. Page through all the pages you need and compare critical areas with the original.
4. If the original had searchable text, search the copy for a word you know appears in it and copy a short passage. With a pure image scan, reducing the size does not add searchable text.
5. For forms, change a test field, save a separate copy, reopen it and trigger the calculations or actions that matter.
6. Click important links and check that they lead to the correct destination.
7. If PDF/A or PDF/UA is required, check the file with a suitable validator.

For a systematic way to compare image quality and document features after reducing the size, see the guide Compare appearance and features.

Use a suitable original copy and an authorized workflow

A typical situation: a portal only accepts files up to a certain size, but your PDF is already digitally signed. Printing to PDF, taking screenshots or converting pages to images may then look like ways out. However, each of these creates a new file in which the signature is visible only as an image, if at all. The original's digital signature is not part of this new file, even if the ink is still visible.

When changes are needed, Adobe refers you to an unsigned original version or to the source file from the person or organization that created the document. Adobe also recommends keeping an unsigned backup before signing (Adobe: Limitations of signed PDFs). This suggests a sensible order: reduce the size first, then sign.

1. Keep the signed file unchanged. It remains the record of what was signed.
2. Ask the person or organization that created or signed the document for an unsigned version or the source file.
3. Reduce the size of this version and check the saved copy as described above. You can do this directly in your browser here: Compress a supported unsigned PDF.
4. Keep the reduced, still unsigned version as a backup.
5. Have the reduced version signed through the designated signing workflow, then check the validation status in the signature list.

This order is a practical approach. It is not legal advice and not a guarantee of validity. Whether the newly signed file meets your recipient's requirements depends on the signing workflow and on the recipient.

If no unsigned version exists, ask the recipient for another way to submit the document. Before that, you can calculate whether your files actually exceed the limits in question with Check upload limits. The guide Reduce PDF size for email explains when a link might be a better fit for an email than an attachment.