Even though work on the internet is publicly available, 'free to view' content is still copyrighted, unless the copyright has expired. Any further copying and sharing, other than for the purposes of non-commercial private study or research (fair dealing), requires explicit consent or a licence.
If you provide a link without permission, it is important to only do so if you are not instructing students to systematically copy the material you are linking to. Links provided as supplementary reading or for information to those who may wish to follow up with further private study are unlikely to infringe copyright law.
You should also try to use reputable sites wherever possible, as you could still leave yourself and the institution liable for secondary copyright infringement, if you provide copies of content which has been illegally posted on the internet in the first place.