Hi,
You aren’t really clear on what documents file path are you looking for?
For example you could get directory where projects are with:
const TCHAR* FGenericPlatformProcess::UserDir()
Or are you looking to just browse the file system anywhere (this could be a problem depending on the system permissions, so needs to accounted for)?
Perhaps the problem you had with FFileManagerGeneric was that of relative vs absolute path?
You are probably best of working with relative paths (relative to the Engine root) so that your code works on all platforms.