So, if you want to search for all pdf file, just type *.pdf in the search box and it will show you all PDF files in results. Similarly you can do for other kind of file types and get the results. If you want to find a pdf file with the install keyword in name, you can search for keyword EXT:pdf. Searching for keyword PDF also works. But what if you want to search for multipe file extensions simultaneously? The process is similar and most of you can already guess if you know much about Windows search operators.
If you want to search for a file name in extensions PDF, CSV and DOC, you can combine the operators like this
file name Ext:.PDF OR Ext:.CSV OR Ext:.DOC
This also gives same results.
file name Ext:PDF OR Ext:CSV OR Ext:DOC
One thing to note that OR will be capitalized otherwise it will not work.