"These files do not open on site"
Correct, there's no pdf reader in the forum software itself (have never seen that in any type of forum). It's a job for the browser (built-in reader). Different browsers deals with pdf's differently, some offer to open immediately, some not. And one can use a standalone pdf reader program installed on one's pc\mobile\tablet.
Have tested all the attached .pdf's, all working.
An example from Firefox, offers 'save' or 'open'. If choosing 'open', it saves it in a temp folder (see address bar above pdf in attach.) and displays the pdf, then deletes the temp file when closing the pdf window. Other browsers may not have the 'open' possibility, then it must be downloaded first, then opened.