Hi Mike,
Yes, you do have to extend your visitor visa if you have a PR application in process and your visa would expire before the process ends. If you applied outland there is no guarantee that you are going to get more time, if you applied inland they extend your visa until they finalize the PR or very close. I suggest you to apply online as the processing time is 50 days comparing to the paper appl. which is 107 days now. You go to CIC website and under the visitor category you will find the needed links for the extention. You have to upload your documents need to be attached to the application and fill out the online form. You can pay the fees as well with a credit card online. I recommend you to enclose the receipt of the pr application, bank account statements, possibly your partner`s letter that she is willing to support you and paystops for a month or a letter from her employmer confirming her salary. Hope it helps!