One application that I have been using for years is Agent Ransack, a fabulously fast free search utility from Mythicsoft.
I have always found the built-in Windows search functionality to be poorly in adequate. So I turned to a third-party application.
Agent Ransack has a lite version which is free for both personal and commercial use.
It features both a basic and expert layout, supports a load of filters and regular expressions and it is blazingly fast.
Searching for all PDF files in my Dropbox, it found 2,568 files from a possible 296,910 items (116 GB) within two seconds. When I tried to do the same using search within Windows Explorer, I ended up having to restart Explorer as it had got too sluggish.
Download Agent Ransack at Mythicsoft.