Q. Can I download/print a textbook that I found in full-text online?

The short answer is no, since that would be violating copyright laws. You can only reproduce a work with the copyright holder's permission.

The rule of thumb is that an item is copyright protected unless it states otherwise. However, under Fair Dealing, you could print up to 10% of the work without violating copyright if you're certain the textbook was posted by the Copyright owner.

What are my alternatives if I'm looking for open source textbooks?

For free materials, you may want to look at eCampus Ontario or OpenStax among others.