If don't mind using a command-line tool for the searching and invoking of your program, you can try Ki:
You'll have to configure Ki to open .pdf files with the program of your choice, however (use ki -p for prompts about it). The program need not be a command-line one, of course.
I mostly use Ki for creative writing on the command-line where there are lots of files within a complex directory structure (using Nano or Less to edit/read them), but you could use it for finding and opening PDFs.
There might be some bugs in the ki -u (when used to install it for the local user or system-wide); so, installing to your path manually might be better.