These two verbs often overlap, but with exceptions. I actually find it difficult to explain how to properly use each one, but let’s say that pensar relates more to the act itself of thinking, while achar refers to a specific opinion you hold. In sentences such as the one you posted, they may overlap:
- O que acha o Pedro dos seus jornais?
- O que pensa o Pedro dos/sobre os seus jornais?
In other cases, it’s easier to see what sets them apart:
- Estou a pensar em ti. (I’m thinking about you)
- Estou a achar em ti. (gibberish)
Achar may also mean to find. This seems more common in Brazilian Portuguese, but it’s also accepted in European Portuguese.