Thank you for the reply. I have set my languages to German and English. These are the only ones I am using.
AI search is still very unsuccessful. For example, I asked how a certain feature of my camera works. Result: No note contains information about this feature. However, when I ask in which chapter of the manual of my camera I can find an information about the feature, it tells me the correct chapter. When I then ask how this feature in chapter xxx of the manual of my camera works, it answers correctly. That is strange and makes the search pretty much useless.
As far as the search in more than one PDF in a note is concerned, it finds notes which contains the information in one of the PDFs. But when I use the search within the note, it shows no result when I display the PDFs as title only. If I display them as one page or showing all pages, it shows the number of occurrences without any indication in which note, and it does not jump to the occurrences in the note. The only way is to open the PDFs one by one in the Mac preview app and search for the text outside Evernote. This happens with scanned PDFs as well as PDFs containing an OCR text layer and therefore its own index. So for me, Evernote search is terrible.