How to tell whether a PDF was modified, and what changed

Metadata

A PDF carries creation and modification dates, the producing application and sometimes an author. These are easy to read and easy to forge, and a legitimate re-save changes them without changing any content. Metadata tells you a file was touched, not whether the document changed.

Hashes

A cryptographic hash of the file changes if any byte changes. Comparing the hash of the file you received against the hash of the file you sent proves whether they are byte-identical. It says nothing about what changed, and two files with identical content produced by different tools have different hashes.

Content comparison

The only method that answers the real question. Compare the text of the version you sent against the text of the version you received. The result is a change list: every insertion, deletion and replacement, with page numbers. An unchanged document produces an empty list. A document that was re-saved by a different tool, with different metadata and a different hash, also produces an empty list, which is the correct answer.

When to use which

• Hash: when you need to prove two files are the same file, for evidence or archiving.
• Metadata: as a quick hint, never as proof.
• Content comparison: whenever the question is "did the terms change", which is nearly always the question.

Try it

Upload the sent and returned versions on the homepage. Documents up to 30 pages need no account.