This bug was previously reported:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=799526

It was closed but the bug still persists in the current version after
the transition from pdftk to pdftk-java. There may have been other bug
reports that have been lost in this way.

Sid Steward-- I’ve got you in copy because I’m not sure if you
normally monitor this bug tracker.

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The "pdftk --help" documentation states that the update_info function
is expecting the same format of data from the dump_data function.
However, it does not work on a variety of files, even with no
alterations.  E.g.

  $ pdftk foo.pdf dump_data | pdftk foo.pdf update_info -
  Done.  Input errors, so no output created.

If the piping seems questionable, even the normal approach breaks:

  $ pdftk foo.pdf dump_data output dogfood.txt
  $ pdftk foo.pdf update_info dogfood.txt
  Done.  Input errors, so no output created.