Comparison with pdftk features:
[x] = done, [?] = maybe, [ ] = to do

[?] Fill PDF Forms with FDF Data or XFDF Data and/or Flatten Forms
[?] Apply a Background Watermark or a Foreground Stamp
[ ] Report on PDF Metrics such as Metadata, Bookmarks, and Page Labels
[?] Update PDF Metadata
[?] Attach Files to PDF Pages or the PDF Document
[?] Unpack PDF Attachments
[ ] Uncompress and Re-Compress Page Streams

Known issues:
- The same page of the same input file cannot be added to the output twice
  with two different rotations (e.g., ``cat foo.pdf 1R 1D`` won't do what
  you'd hope).
