If a PDF file contains binary data, as most do (see 7.2, "Lexical conventions"), the header line shall be immediately followed by a comment line containing at least four binary characters–that is, characters whose codes are 128 or greater. This ensures proper behaviour of file transfer applications that inspect data near the beginning of a file to determine whether to treat the file’s contents as text or as binary.
Since most PDFs do contain binary data the best default behavior would be to always add the marker comment.
Since most PDFs do contain binary data the best default behavior would be to always add the marker comment.