Some are available to download in full, free of charge, but they seem to be in the minority. Each trial will indicate the cost and how you can pay for it, online.
On the other hand, there is a way of looking at a summary of a clinical trial free of charge; it’s called an abstract, and they can be found on the USA’s National Center for Biotechnology Information (NCBI). Their website is www.ncbi.nlm.nih.gov. No account is needed for access to this website.