Actually their are no surviving sources contemporary with the voyage of Pytheas that describe the voyage. And the accounts that have survived are very poor.

The main source is the writer Strabo in his Geographia written in the late 1st century B.C.E. to the early 1st century C.E. Sadly not only are Stabo's references to Pytheas brief, but Strabo seems to have held Pytheas' and his account in contempt.

The two other sources are Pliny the Elder's Natural History written c. 60-75 C.E. Further information on Pytheas' voyage is in the Universal History of Diodorus, written in the 1st B.C.E.

Pytheas' voyage appears to have occurred between 322 and 285 B.C.E. The earliest surviving accounts of the voyage are 2+ centuries later.