PdfDocumentId Property

(Required if an Encrypt entry is present in the file trailer; optional otherwise; PDF 1.1) An array of two byte-strings constituting a file identifier (see 14.4, "File Identifiers") for the file.

Because the Id entries are not encrypted it is possible to check the Id key to assure that the correct file is being accessed without decrypting the file.

Although this entry is optional, its absence might prevent the file from functioning in some workflows that depend on files being uniquely identified.