It is not clear from the PDF ISO standard document (PDF32000-2008) whether a comment may follow the startxref keyword:

startxref
Byte_offset_of_last_cross-reference_section
%%EOF

The standard does seem to imply that comments may appear anywhere:

7.2.3 Comments

Any occurrence of the PERCENT SIGN (25h) outside a string or stream introduces a comment. The comment consists of all characters after the PERCENT SIGN and up to but not including the end of the line, including regular, delimiter, SPACE (20h), and HORZONTAL TAB characters (09h). A conforming reader shall ignore comments, and treat them as single white-space characters. That is, a comment separates the token preceding it from the one following it.

EXAMPLE The PDF fragment in this example is syntactically equivalent to just the tokens abc and 123.

abc% comment ( /%) blah blah blah
123

Comments (other than the %PDF–n.m and %%EOF comments described in 7.5, "File Structure") have no semantics. They are not necessarily preserved by applications that edit PDF files.

If they are allowed to appear after the startxref, parsing the file becomes more difficult because you do not know how far to back up from the %%EOF comment to start parsing to find the byte offset.

Any ideas?