Keep in mind I'm visa exempt, so it's a little easier for me to extend my stay.
I sent in copies of my pp picture page and stamps plus 3 paystubs for my husband. I also had residency pending. In addition, I wrote in the "why do you want to extend your stay" field I was waiting for residency and my husband and I wanted to stay together. I offered copies of my receipts for proof if CIC needed it. Apparently they didn't, because they never asked for anything. (It wouldn't be that hard to find my PR application because I have a very unusual last name, so I'm guessing they cross referenced and approved.)
I asked for 14 months to finish out my PR based on the time CIC has posted on their website. CIC approved the entire 14 months.
If you have a PR application pending, make sure it reference it and offer proof. If you want to stay for other reasons, you'll need to treat it like a brand new visa application and include all the same proofs as before.