What program do you recommend to share files over WiFi between your PC and phone?

If you create a shared folder using your OS (which in Windows creates an SMB share) you can then use any FOSS file browser app with SMB/samba support to move files back and forth directly. Many of them support that protocol since it's common. Amaze is one such app that I've used for this. You'll need to set the shared folder to allow write privileges if you'll be sending things from your phone, and you might want to protect it with a username/password for security.