Search inside some Text-only PDFs does not work?

Operating system

macOS

Joplin version

3.5.13

Desktop version info

Joplin 3.5.13 (prod, darwin)

What issue do you have?

I have searched already the forum but couldn't find a clear answer .. it seems like Joplin can search OCRd Images inside a PDF, but it would not find Text inside some PDFs that seem to be pure Exported Text files. But it is inconstitent - there seem to be document that work and other that don't. What could influence the searchability of a PDF document ? ( The particular one is a signed document )
The document is coming through an import from Evernote into Joplin.