Glossary
What is a document hash?
Short answer
A document hash is a digital fingerprint of a file: a hash function such as SHA-256 reads every byte and outputs a 64-character hexadecimal value. Change a single comma and the hash changes beyond recognition, while the same file always gives the same hash. E-signature services record the hash so anyone can later prove a copy is the exact document that was signed.
Updated 25 October 2026
- Document hash
- A document hash is a fixed-length value computed from a file's contents by a cryptographic hash function such as SHA-256. Any change to the file, however small, produces a completely different hash, so matching hashes show two files are identical.
How hashing works
A cryptographic hash function takes input of any size and returns output of fixed size. SHA-256, part of the SHA-2 family published by NIST in FIPS 180-4, returns 256 bits, usually written as 64 hex characters. A 40-page lease and a one-line note both produce a 64-character value.
Three properties make it useful. It is deterministic, so the same input always gives the same output. It is one-way, so you cannot rebuild the document from the hash. And it is collision resistant, so finding two different files with the same SHA-256 hash is not practical with current computing.
Why hashes matter for signed documents
A signature is only useful if you can show what was signed. If the parties later hold two versions of a contract with different payment terms, comparing each against the hash recorded at signing settles which one is real.
Digital signatures also sign the hash rather than the whole file, which is why a hash change breaks the signature.
How Sign PDF uses SHA-256
When you send a document, Sign PDF records the SHA-256 fingerprint of the original and prints it in the certificate of completion. The audit trail entries are also hash-chained: each entry includes the hash of the one before it.
Our verify page at signpdf.ai/verify fingerprints any copy you drop in, inside your own browser, and compares it with the sealed original. The file never leaves your computer; only the comparison happens.
What a hash does not tell you
A hash proves a file is unchanged. It says nothing about who created it or who signed it. That comes from the audit trail and the digital signature. A hash is also sensitive to harmless changes: re-saving a PDF in another program, even without visible edits, usually changes its hash.
Questions people ask
Can two different documents have the same SHA-256 hash?
In theory, yes, because there are more possible files than hashes. In practice no one has found a SHA-256 collision, and finding one deliberately is beyond current computing.
Is a hash the same as encryption?
No. Encryption can be reversed with a key. A hash cannot be reversed; it only lets you check whether two inputs match.
Why did my PDF's hash change when I only opened it?
Opening should not change it, but saving it again, even without edits, often rewrites metadata. Keep the original signed file untouched and share copies of it.
How do I check the hash of a Sign PDF document?
Drop it into signpdf.ai/verify. It computes the SHA-256 fingerprint in your browser and tells you whether it matches the sealed original.
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