Hi Samira,
No of course it will not cause any problems, as a PR he should be "residing" in Canada, a visit won't make him lose the "residing".
I'm a PR myself and I stayed for 4 weeks. Cic doesn't exactly mention how many days short visits mean, but I think it's about a month. Cuz when I think about it, when someone is residing in Canada, he/she can't just leave the country for 2-3 months because usually he/she has a job or school or sth. So that's why I assumed 1 month is normal! That's all my assumption and based on what I read on other forums.
The part that I'm sure about is that a visit won't do any harm to an application. I remember asking the same question before leaving and Sohrab (who works for cic) assured me that my assumptions are right!