+2 for Andy @ Tramuntana Tours. He'll sort you out a hire bike, route maps and / or guiding / uplifts. He's based in Soller, which is a better bet for mountain biking than Pollenca. I've stayed (and ridden) in both and Pollenca has a very good bike hire shop - might have to look back through my threads for the name but it's Pro something or other. It's spectacular for road riding - out to the lighthouse at Formentor is staggeringly good.
Pollenca is a fair way (through the mountains) from Soller, so it would be one or the other - though if you fancy a great road ride, the MA-10 (I think?) through the mountains, taking in Lluc, several tunnels, Fornalutx, Soller and the like, is stunning, and you can then come back along the flats of the plain in the middle of the island. They love cycling / cyclists there - and there are loads of mapped, quiet road routes available to you.
The mountain biking from Soller is to be recommended, but what comes down, has to go up...


Whichever you do, you'll have a great time!