Uncompress?

No.

compressed

That's what a "backup" is for.

compressed

There is an error in the QB documentation. It uses the word "compress" where it should use the phrase "delete permanately."

Load the backup you were required to make before you did the "compress."

Yes. The actual phrase should be "... compress (the size) of your database by deleting historical data..."

If we really want to get nit picky, Intuit uses the term condense. When the user condenses their data file, individual transations for each month being condensed are replaced with a single net amount.