Yes, you should get a certificate back with the bottle. The certificate itself simply has the date of test, serial number of bottle, name of testing company, etc. and whether it passed the test. It's a good idea to hang onto this in case you sell the bottle, but most paintballers are probably happy enough if the label on the tank itself has the right date
H-PAC also include a test document with the certificate with a little more detail about the tests done (visual check, valve/threads, weight, hydro test etc.) and the results.