If you mean a link that is referenced the non-NetSuite way then no it’s not possible (as fr as my experience with external links in PDF templates goes) to reference an external link that way and expect it to work in a PDF template. For example if you want to use Google Fonts the standard way is to grab the link and use it in an HTML file. For NetSuite you would need to download the font file, upload it to the file cabinet and reference it via the URL field.