My Windows search function recently once again stopped indexing pdf files, probably associated with an upgrade to Windows 11. Previously this was fixed by downloading the file PDFFilter64Setup.msi from the Adobe site, but that download no longer works, and there is nothing useful from Adobe or from Windows help. After much searching I eventually found a Wikipedia article with a working link:

https://en.wikipedia.org/wiki/IFilter

The working link is:

https://web.archive.org/web/20210227135128/https://supportdownloads.adobe.com/thankyou.jsp?ftpID=5542&fileID=5550 –  Adobe PDF iFilter for 64bit Windows systems

Note that the link listed at Note 1 in the Wikipedia article didn’t work.

For details of installing the file, and updating the search settings, see Indexing pdf content.