This will force the file to be downloaded to be viewed. Check the box next to Disable in-line preview.
This only applies to links from withing the Rich Content Editor though, not links from Modules. If cases like this the file needs to be downloaded to be viewed correctly so disabling the InLine preview option can help with that.
I have occasionally seen cases where a PDF does not display correctly in the Canvas Doc View, such as if the PDF is password protected. Are you sure the links are active in the PDF file before you uploaded it to Canvas? I just tested a few pdfs in my courses and the links in them work fine when viewed with the DocViewer in Canvas and if I click the link to have it download.
