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Exporting a PDF in the issue view results in a perfectly viewable PDF in Acrobat Reader DC 2019.008.20080
All readers tested here (zathura on linux, firefox on linux, chrome on windows) show the PDF perfectly, and copying text from it works as expected.

However, copying text from Acrobat Reader DC results in "rectangles" (unknown glyphs) when pasted into whatever application (Outlook, Word, Editor, Notepad++). Changing the encoding in Notepad++ also did not result in a usable text.

Searching through rbpdf and tcpdf issues did not result in anything useful. I also tried changing the font from freesans to DeJaVuSans but that did not solve it either. I don't know where to look next..
If that helps: A recent version of tcpdf generates a usable PDF with LimeSurvey but the process is pretty different (PHP, not ruby, completely different content, ..)

This might be related to #26034.

A sample PDF is attached.