Guide
Open a Reaper Project in Digital Performer
Digital Performer can import an OMF file, which is what the converter makes from your Reaper project.
Digital Performer can export and import OMF files directly, no extra plugin needed — use the import function and pick the .omf file.
The converter embeds the audio in the OMF, so there are no separate audio files to track down.
You can convert your Reaper project to OMF for free here.
Going the other way? Here's how to open an OMF file in Reaper.