For the document information dictionary properties CPDF is limited to the following properties:

• Title
• Author
• Subject
• Keywords
• Creator
• Producer
• CreationDate
• ModDate
• Trapped

The relevant section of the PDF (1.3) spec is 8.2 Document Information Dictionary. The section does not explicitly indicate that custom values are allowed, but does implicitly allow it with the following statement:

Note: Although viewer applications can store custom metadata in the document information dictionary, it is inappropriate to store private content or structural information there; such information should be stored in the document catalog instead (see Section 3.6.1, “Document Catalog”).

Additionally, the implementation notes for 8.2 state the following:

94. Acrobat viewers impose a limit of 255 bytes on any string appearing as a value in the document information dictionary.

Though no size limit is apparent in the spec itself.

Originally posted by @RoDiLki in #2447