Remember that as long as they're not a PR, the PA will be seeking entry into Canada as a visitor. That means showing that at the end of their allowed visit (usually 6 months) they will return to another country.
To make a strong case, be prepared to show not only a return ticket, but evidence such as a residence, work, health insurance, a means of support, and basically a life outside of Canada.
The fact that you will be in Canada and that there's a PR application in process will show the border guard that there's a very big chance your partner won't leave at the end of their allowed stay as a visitor, should the application not be completed, or even refused. They have the option of offering a visitor record which means that your partner would have to report their departure directly to CBSA at the end of their allotted stay, but this is up to the border guard.
I'm the PA in our application and I'm currently in Canada visiting my husband, and have been let in without problems or many questions any time I've crossed the border, but I've only ever stayed here for a few days or weeks at a time, never many months, and always with evidence of strong ties to my home country (work, memberships, other family).