We have two different behaviors when clicking on a link:
First case:

• edit a content
• add a link to a PDF in ressource explorer
• save
• Click on the link
  => the browser is opening the document with its own pdf viewer (which is nice)

Second case :

• add attachments to the page
• add the service to display attachments
• click on the PDF in the service
  => the browser is downloading the file

If you compare both URLS, they are not the same, on is containing something like ?download=true.

It could be nice to have the same behavior on both cases. The first case is better for the user because the viewer give you the possibility to view with less click, download, print and probably store the pdf in a temp directory so the user does not need to clean...