AFAIK it's under GPL because it contains libraries from xpdf and maybe
a few others which were already under GPL.
1) Thanh made his work GPL'd independently of the code from xpdf (which
is only used for reading pdf files, not writing).
2) No xpdf or other third-party code would be relevant to the
implementation of \pdfstrcmp et al.
Thierry, my comment/offer stands. -k