In some cases (sometimes), I.M.H.O.(g), using the HTTP, you really need to force visitor system (or UA) file downloading instead of reading or open. Even the skillful Dan R. Tobias (with full respect), in his subsection ‘Forcing files to download, run, launch specific applications, etc.’ [http://webtips.dan.info/force.html] wrote ‘You can't do that.’ and ‘Let visitor to click by right-mouse button on the link if he want to save it.’ It is the way widely using by nearly all sites which offers free (or share-ware) programs as Tucows, download.com etc.
If you want to force visitor to download (save) your PDF file instead to open it from your location you can do that on the following way:
[(Force) download sample.pdf (5.40 KiB)]