Is there a good and trustworthy bitcoin exchange where I can trade my bitcoins for CAD and then send the money in my PayPal account instead of a bank transfer? Thank you
Short answer: No.
Long answer: PayPal does not allow Bitcoin exchange accounts. Bitcoin exchanges pop up like mushrooms these days so a new one might fly under PayPal's radar and get an account going for a while. But it will eventually get shut down.
So trustworthy exchanges won't try to use PayPal, as they know their accounts will be seized. And you shouldn't trust those that are using PayPal because they're operating with high risk. Knowing the high risk.
PayPal's CEO talked about Bitcoin as a funding option on PayPal in an interview during the last couple of months.
Until that happens the answers above are unlikely to change.
What might exist is a service that lets you pay for charges with Bitcoin and then the recipient gets to withdraw the proceeds of the sale using PayPal. So you would simply need to sell to yourself.
For example, I see that Fiverr accepts BItcoin for payment: http://blog.fiverr.com/fiverr-now-accepting-bitcoins and I believe they offer withdrawal via PayPal http://www.youtube.com/watch?v=px_eYHCgGZ4. So you simply would create two accounts, list a task, and transact with yourself. Obviously the account for buying (and paying with Bitcoin) would need to be something that you wouldn't want associated with your real identity. Additionally, you probably want to do the listing with one IP address and the buying on another just in case they check for that kind of stuff.