What you are looking for is described here:
http://fossil-scm.hwaci.com/fossil/doc/trunk/www/embeddeddoc.wiki
In short, you would write <baseurl>/doc/tip/<filename>
you can read <filename>
above to be the path to your file, exactly as it is shown after "file" in the artifact content page. In other words: through the UI you go to "files" and navigate the the files you want link to. Once you are at the file, the header of the page will be "Artifact Content" and show you the as a full path after "File" (right at the top, after the artifact ID)