What does Checksum mean for your PDF?
Checksums answer one question precisely: is this the same file I had before? Two copies of a contract with the same SHA-256 are byte-for-byte identical; a different value means something changed, though it does not say what.
SHA-256 is the current standard. SHA-1 and MD5 still appear in older systems and remain fine for spotting accidental corruption, but not for proving nothing was tampered with.
Note that re-saving a PDF — even with no visible edit — rewrites its bytes and changes the checksum. That is the behavior, not a fault.
How do you actually compare two files?
Your computer can do it without installing anything. Run the same command on both copies and compare the strings — if the first and last few characters match, so does the rest.
- macOS and Linux:
shasum -a 256 document.pdf - Windows PowerShell:
Get-FileHash document.pdf - Windows Command Prompt:
certutil -hashfile document.pdf SHA256
What this proves is narrow and useful: the file you received is the file that was sent. What it cannot tell you is what changed, when, or whether the change was innocent — for that you need the two documents side by side. (Checksums are case-insensitive hex, so a tool that prints uppercase is showing the same value.)