DT does not see text in text layer of pdf

While indexing a lot of pdf documents, I keep encountering files where DT’s log shows “No text”. Those files also can not be searched in Preview, marking a text shows the structure of the words, but when the text is copied and pasted, nothing shows up in the target document.

However, I can both search and/or copy text from this document e.g. with Acrobat Reader DC, calibre’s ebook-viewer or from the command line with pdftotext.

Running pdfinfo or qpdf --check on those files does not report any errors.

What could be done to make those pdfs behave correctly, preferably with keeping the links and markups contained in the files?

PS.: This on OS X 10.11.6

Please start a Support Ticket with at least one problematic PDF attached.