If the PDFs are text for the most part, then this plugin may be of interest, the Resource Search Plugin.
It doesn't OCR anything, but can accesses text within PDFs, thus making them searchable. However, it will not scan text from images.It might be a partial solution.
There are a couple of other plugins that may be of interest; if you go to Joplin Tools > Options > Plugins and type OCR in the search box you'll find them.