Always link to the existing digital version of the text (e.g., article in a database). Most license agreements require use of the stable or persistent link provided and forbid downloading of content for re-posting in a CMS/LMS or electronic course reserves system.
Digitize hard copy texts and post electronic version in secure site accessible only by enrolled students within bounds of fair use.
Seek permission of not fair use.
Look for Creative Commons or open access content such as article versions that have been posted in an open repository according to the archiving policy of the publication. Utilize a service such as Unpaywall to help locate open access versions of scholarly material.