Try the <pdf src="[KNOWN URL TO A FILE]"></pdf> to see if that file can be read without the "Available without login" box being checked. If you can access that file without it being "Available without login", then you may have hope. If you can't - in order to complete your goal, you'd need users to open the file in the file cabinet and mark that checkbox each time they add a file to the transaction, which might cause many other headaches