In the export currently translated PDF files are named xyz.pdf, analogous to all other file formats.

This is wrong, since a PDF file is not exported as PDF any more on file export, but as txt file.

Therefore for PDF files the file extension .txt must be added to the files, so that they match the pattern "xyz.pdf.txt".

The background is:

Anyhow regarding the PDF export: The file exported by you in your zip is
actually fine, except that it should have been named "ORIGINAL.pdf.txt".
Okapi that does the file format conversion for translate5 can handle
PDFs for import but is not able to reexport PDF. Instead it exports a
plain text file with the contents of the PDF. Therefore this is indeed a
translate5 bug - we have to rename the file and add "txt" to the file
extension. We will fix that soon.