purgeable ok?

Optimizing the storage only deletes temporary and cache files which are going to be deleted at some point anyway...it won't touch your documents. The "purgeable" space may also include hidden APFS snapshots usually associated with Time Machine backups (or from other third party backup software)....these backup snapshots are deleted some time after transferring the backup to external backup media.

Here is an Apple article which mentions "Purgeable" space which I quote here:

Get detailed information about a disk in Disk Utility on Mac - Apple Support

* Amount of space available, which can include both free space and “purgeable space”—or space that macOS can free up when needed by removing files from your computer (you can’t manually remove the files that are designated purgeable, but macOS removes them as space is required)

Purgebale is essentially temporary files and caches created by the OS in its normal operation. The OS will manage this material on its own, deleting it if it needs space, but you can do so also.