Hi Solange,
The reason why you're not seeing the campaign variable defined is because pdf files can't really send image requests. On standard web pages, appmeasurement libraries are loaded, which in turn defines the logic to capture that query string parameter and send it to Adobe's servers. If you're wanting to define the campaign variable when link clicks to a PDF file, you'll want to do it on the page prior to opening the PDF.
If you're using a legacy implementation, use an s.tl() call. The link type would be lnk_d, and be sure to set s.linkTrackVars to contain campaign.
If you're using DTM, use a event-based rule to listen for any links to .pdf files. Then in actions, you can set the campaign variable to what you'd like.
