Searching an open PDF file in Acrobat is easy.
Just click the little binoculars icon on the File
toolbar, enter your search term and conditional
filters (checkboxes) and click Search. What is
not commonly appreciated, however, is that you can
search an entire folder of PDF documents, none of
which have to be open in Acrobat. To do so,
click the Search icon (the binoculars - or click
Edit > Search, or press Shift-Ctrl-F). Under
the "Where would you like to search?" area of the
search pane select "All PDF Documents in" then click
the drop-down and choose "Browse for
Location..." A Browse dialog box will appear
allowing you to navigate your file system to the
folder you want to search (this is another good
reason to store all PDFs in subfolders of their
own). Select the folder, enter your search
term, apply conditions (if any) and click Search.

Acrobat will report search results by document:

By clicking any of the contextual search results
entries Acrobat will open the document and jump to
the location (which will be highlighted in the
document) within the PDF file.