fix: encrypted PDFs with EncryptMetadata false fail with invalid password

[mdz]

Summary

initEncrypt never read the /EncryptMetadata entry, so PDFs encrypted with the standard security handler (V=4, R=4, AES-128) that set /EncryptMetadata false failed with encrypted PDF: invalid password, even when the actual password is empty.

Per PDF 32000-1:2008 §7.6.3.3, Algorithm 2, step (g): security handlers of revision 4 or greater must pass 4 bytes with the value 0xFFFFFFFF to the MD5 hash after the document ID, if document metadata is not being encrypted. This step was missing entirely, so the derived key never matched the file's stored U value.

This encryption combination (AES-128 content encryption with cleartext metadata for indexing) is common in e-signature/contract-export tools (DocuSign, Adobe Sign, PandaDoc, etc.), so any signed PDF exported that way failed to open.

Originally reported at ledongthuc/pdf#82; filing here since this fork is the actively maintained one.

Changes

• read.go: implement step (g) of Algorithm 2 — append 0xFFFFFFFF to the MD5 hash when R >= 4 and EncryptMetadata is explicitly false.
• read_test.go / testdata/encrypted_metadata_false.pdf: regression test using a minimal repro PDF (empty user/owner password, V=4, R=4, AES-128, EncryptMetadata false).

Test plan

• go build ./...
• go vet ./...
• go test ./... — all pass, including new TestOpenEncryptedMetadataFalse

[coderabbitai]

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