The library is already in the project directory? Are you sure you don’t want to just move it into the lib/, add a library.json with the -I, -L and -l build flags and call it done? That would be the canonical way of doing it – creating a proper PlatformIO libray for it. No need for build_flags hacking in the platformio.ini then, too.
There’s a details tutorial for that at How to generate and use pre-compiled objects - #2 by maxgerhardt.
Anyways, since all the paths in the -I and -L flags are already relative to the project directory, have you tried
build_flags =
-Ipd_open_library
-Lpd_open_library
-llibPDlib.a
?