I know that the Vikings came into contact with Muslims when they raided Islamic Spain, but that's as far as my knowledge on this matter goes. Did Muslims come in contact with any other pagans in the lands that they conquered? Any Spanish pagans, Italian (Sicilian) pagans, any in the former Byzantine territories (Graeco-Egyptian pagans, any Hellenic-Roman pagans in the Levant), etc? If yes, how did they interact with them? Were they persecuted for a bit and then given "people of the book" status like the Zoroastrians? Or were they completely wiped out from the beginning?
This wasn't in an area that they conquered, but the 10th century muslim explorer ibn Fadlan also met some Vikings in Russia, and interacted with them briefly. There's very little theology or so forth as he was an explorer, not a theologian, but he does have a few opinions on them and witnessed some religious burial rites of theirs. It's described by this user's answer to a different question: https://www.reddit.com/r/AskHistorians/comments/5tlx1e/how_did_the_vikings_view_islam_as_compared_to/ddolj0u/
In an area where they did conquer, a fruitful avenue might be to ask or research about the Hermeticists of the city of Harran, who successfully appealed to be considered People of the Book and were termed "Sabians" (but were probably not the same Sabians as Muhammad's Sabians?). While you might or might not consider Harran/Carrhae "European" as it's at the eastern edge of Turkey, the Corpus Hermeticum (the "Book" of "People of the Book" for them) was a well known non-Christian text in Europe as well. I don't know enough about them to give a satisfactory description besides mentioning that they existed, and I can't find any useful prior answers on them.
Michael McCormick's Origins of the European Economy is probably your best bet for finding the answer to this. He looks at numismatic evidence to trace the presence of Arabic coins in Europe, discovering that there was a flow of goods along the Amber Trail, a route which had dominated in prehistory and through the ancient world, but historians had assumed trade died off in the early medieval period.
McCormick doesn't directly discuss the contact between pagan Europeans in Eastern Europe (Poland, Lithuania, pre-Christian Hungary), but my best guess is that there was some contact, but it was distant and is only evident in some archaeological remains.