Another US applicant here. I am currently on a post grad work permit and traveled back to the US no problem after submitting my application. As long as you can validly re-enter Canada there should be no issues, I opened my passport to my work permit like normal, he asked what kind of work I do I told him and said I was waiting on my PR application. He says "oh yeah those can take a while" then waives us through.
Any reason they wouldn't let you back into Canada would probably also be reason to decline your PR if you were sitting here not traveling anyway.
The only time I have heard you should not travel outside of Canada if at all avoidable is between the time of receiving your confirmation of PR after your landing interview and when you get your PR card. You can use the CoPR when crossing by land, but you have to file additional paperwork if traveling by air once you leave Canada, so really just a bit of a hassle but you can still do it.
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I am more familiar with the US immigration than here but I feel like there is a lot of overlap. So correct me if I'm wrong. I know that in the states, if you were adjusting your status, unless you came in on a visa that gave you residency status (even if temp), like student visa or work visa, you couldn't travel. However, if you did come in on such a visa, you were permitted to come in and out of the country. Coming in on a visitor status meant non-residency status, which meant that upon your re-entry, you could be denied entry for any reason, whatever they choose, since there is no right of entry of visitors. Therefore, you'd be abandoning your status.
I wonder if you were able to travel in and out of Canada because of your post-grad work visa? This allows you re-entry rights, more so than a visitor. My husband came in as a visitor. Although, I would assume you are correct (only makes sense) that so long as you would be granted entry into Canada as a visitor, there is no reason to not be granted entry just because you have a PR application pending, he still doesn't have a right of re-entry on a visitor status. Which then leads me to wonder, now that he has been given a work visa through our PR application, would that give him (stronger) entry rights pending application?