Generally speaking, you would have interviewed with the company hiring you in Canada and would know the job offer is real. Did you pay money in exchange for the job offer or are you being asked to pay money? If so, it's almost certainly a scam.
Note that a job offer letter isn't enough to apply for a work permit - the employer also needs to provide you with an approved LMIA.
Are they asking for money, do the websites mentioned work, did you get the job through an agency, is the compensation much higher than it should be, etc
If you are asking this question I am inclined to think it is a fake job offer.
