Choose File > Save project as, tick Create subdirectory for project and Copy all media into project directory, and save into the backup location. REAPER writes a new folder holding the .rpp and a copy of every media file the project uses, wherever those files lived before. The original files stay where they are. A .rpp copied on its own only points at media by path, so it opens elsewhere with empty items.

Back up a project

The save dialog offers three ways to handle media:

• Copy all media into project directory: copies everything in. The safe choice for a backup or a handoff. Add Convert media and pick a format to convert while copying, for example FLAC or OGG for a smaller session to send.
• Move all media into project directory: moves the files instead of copying them. If some media came from a sample library, tick the option to copy rather than move files that are not in the old project's media path, so you do not empty your library.
• Neither: saves the .rpp only.

Plugins are never copied. JSFX ship with REAPER; VST, AU and CLAP plugins must be installed on the other machine.

Version a project and roll back

Every time you save an existing project, REAPER keeps the previous version as .rpp-bak beside it. Open the .rpp-bak to go back one save.

For more history, open Preferences > Project and set the backups when saving:

• keep the previous version as one .rpp-bak, or
• keep all previous versions in one large .rpp-bak, or
• keep each previous version as a timestamped .rpp-bak,
• with Save timestamped backups to Backups project subdirectory to keep them out of the way, and a limit to the most recent copies or unique days so the folder does not grow forever.

The same page has Auto-save to timestamped file in project directory for crash insurance. For a version you name yourself, File > Save project as with a new name.

Send a project to a collaborator

Save project as with Copy all media into project directory to a new folder, zip it and send it. The .rpp is plain text, so it is small and survives email, and REAPER opens projects from older versions. List the third-party plugins it uses. When a plugin is missing, REAPER warns on open and keeps the FX in the track's chain, unavailable until the plugin is installed.

Render stems for mixing or mastering

1. Select the tracks to export.
2. Choose File > Render.
3. Set Source to Selected tracks (stems): one file per selected track, post FX, post fader and post pan, including anything routed into that track. Master mix + stems also writes the full mix in the same pass.
4. Tick Render stems prefader if the mix engineer should get your tracks without your fader moves.
5. Set Bounds to the entire project so every file starts at zero, and use the $track wildcard in the file name.

Doing it in Aden

REAPER is one of the formats Aden reads completely, because the .rpp is open text.

• Your projects, listed. The desktop app finds .rpp files on your disk and lists them on Home. On macOS it marks the one open in REAPER with a green dot, because REAPER keeps the .rpp open while you work.
• Sync the project folder to a track (Max plan). Every save is pushed. A collaborator clicks Pull for the latest files; a pull never overwrites their local edits, it saves the incoming file as a versioned copy beside theirs. Every synced file keeps its version history, downloadable from the track's Project files page.
• A check before you send. Send as a transfer zips the project folder into a link that expires in 30 days, and tells you to copy all media in first when it holds little more than the .rpp.
• Read and play it on the web. The Project files page shows tempo, every track with its volume, pan, mute state and folder, the FX chain (VST, AU, CLAP and JS) and every media file. Press play for a sketch of the arrangement: audio items play your synced media, looped items repeat, MIDI plays through a stand-in sound. Effects and automation are not played.
• Edit and record together. Move or delete items with everyone on the page; Save writes the change into the .rpp as its next version, in the file's own line endings. Vocal takes recorded in the browser are written in as items on a new track at the bar you sang them, so they are there when you pull.
• Plugins before a session. In a live session, the desktop app reads the plugins a REAPER project loads and shows each person which ones their machine is missing, on Mac and Windows.
• Feedback on the bounce. Render the mix and the desktop app notices it: named after a track you have, it offers it as that track's next version. Send a share link and people comment at a timestamp or on a section of the waveform, with no Aden account needed on a link with comments turned on. Send for review asks them to approve it or request changes.
• Stems on a version. Upload your rendered stems to the version's Stems tab to play them together under the mix.
• Aden inside REAPER. The desktop app installs Aden as a VST3, Audio Unit and CLAP plugin on macOS and Windows. Put it on the master to see the track's notes, lyrics and session without leaving REAPER.

Sources: REAPER User Guide 7.81 (saving projects, project backups, render source options), REAPER documentation. See also how other DAWs version projects.