Eric, the error you have linked to (TCPDF ERROR: This document probably uses a compression technique which is not supported by the free parser shipped with FPDI.) is not related to this font issue. The 'compression technique' error is related to difficulty reading the original PDF, this font issue (as far as I can tell) relates to problems when trying to prepare to write to the destination PDF (buy which may be triggered earlier than that).
(As an aside, thanks for flagging-up MDL-43593 to me - it is nice to see how my original code has been improved).
In this case, the bug is a bit strange - TCPDF is loading a font definition file (which certainly exists, as there would be a different error if it wasn't found), but then finding that important parts of that file are missing. This is very odd, as the copy of 2.6 I have shows that file with all the expected parts intact.
Could someone who is having this error please locate the file on the server that the error message mentions (i.e. /something/or/other/lib/tcpdf/fonts/helvetica.php) and attach it to the forum, so I could take a look at it? If you haven't done so already, could someone also create an issue in tracker.moodle.org and post a link to it here?