Securing a job in Canada while living overseas can be a challenge because you are thousands of miles away and you cannot attend in-person interviews. Also, employers often want new hires to start working two weeks after they have accepted a job offer.
It will be easier for you to secure a job when you are physically in Canada because you will be accessible and available to them when they need you. Having said this, obtaining employment before landing in Canada it is not unheard of, and resilience is paramount during the job-hunting process.