Questions answered in this file

Does a matching hash prove authenticity?

No. It supports a byte-level comparison to the baseline you recorded. Authenticity and truth require separate provenance and source assessment.

Why did my annotated PDF get a new hash?

Annotations, OCR, bookmarks, and re-saving can change bytes. Keep that derivative separate and record its relationship to the original.

Which algorithm should I record?

Record the exact algorithm used; SHA-256 is a common practical choice. Comparisons must use the same algorithm and complete value.

Should a mismatch be called tampering?

Not without further evidence. First check file identity, version history, intentional edits, transfer problems, and the comparison method.

Can a hash replace a chain-of-custody record?

No. A hash is one integrity tool, not a complete record of who controlled a file or a legal evidentiary conclusion.