It is often useful to search for a phrase inside multiple PDF files. You can search inside all files in a folder. I will demonstrate this feature in Adobe Reader, Foxit Reader, and PDF-XChange Viewer. The basic idea is the same in all three software. Instead of searching using the small find box which you can invoke using CTRL+F, use a search pane which can be invoked using CTRL+SHIFT+F. The same shortcut works in all three software.
In Foxit Reader, click on the Folder Search Icon located next to the Find Box. As mentioned earlier, you could also use the shortcut CTRL+SHIFT+F.
In the where do you like to search? option, choose All PDF document in.
Clicking on the arrow next to the Search Box gives access to more search options. This is worth taking note of since it is easy to ignore that arrow thinking that it would just show the previous search keywords.
Adobe Acrobat Reader DC
Choose the Advanced Search option from the Edit menu.
This would open a search window.
Click on the Show More Options link at the bottom of the Window to see more search options. Adobe Reader has quite a large number of additional search options.
From the toolbar, click on the icon that looks like binoculars.
You could also choose the search option from the Edit menu.
Here the additional options have a separate tab.