If you notice that Windows search does not return any PDFs as hits, even though you are sure that your search term appears in such a file, check the search settings.
To do this, open the Start menu, search for a term, then click the three dots at the right-most edge of suggested search terms across the top of the interface — next to your account icon. Enter the indexing options and then click on the “Advanced Indexing” option, followed by the “Advanced” button to bring up a new dialogue box.
Switch to the “File types” tab and scroll down to the “pdf” entry. It should be ticked and the filter description next to it should be “Reader Search Handler”. Tick the entry and make sure that the option “Index properties and file contents” is activated under the list. If this is not the case, adjust the settings accordingly and confirm with “OK”.
Next, you need to rebuild the Windows search index. To do this, open the “Indexing options” window again and click on “Advanced”. In the “Index settings” tab in the “Troubleshooting” area, click the “Rebuild” button next to “Delete and rebuild index” and confirm the message with “OK”. When the index is subsequently created, the PDF files not previously included are added to the index. Caution: Rebuilding the index can take several hours or even a whole day!
The search function in Acrobat Reader, as in other PDF viewers, works independently of the Windows search. After you have pressed the key combination Ctrl-F in the Reader, you can search for individual words in the currently loaded PDF file. It is also possible to search through several PDFs in a folder for an expression by clicking on the three dots and selecting “Advanced search”.