To bring this discussion back to the original question: no, there is
no way to completely prevent someone from downloading a copy of a pdf
if you make it available for viewing on your site. The web simply
isn't designed that way (for good reason).

There are other approaches you can take, but figuring out the most
suitable one depends on what, exactly, you're trying to do.

For example, a lot of photographers must have an online portfolio to
promote their work, but they're wisely concerned about unscrupulous
people stealing those images and selling them.

Some photographers choose to embed the images in a Flash movie. This
stops people from being able to simply drag a copy onto their own
computer, or from copying and pasting the image to a local system.
However, when that image is displayed on screen, it is a trivial
matter to take a snapshot of the computer screen and then crop it to
show only the image. Harder to do? A bit. Will it stop people from
stealing images? No. Will you lose sales because some visitors can't
view Flash files properly. Maybe.

So photographers have turned to watermarking their images; essentially
destroying the image so it can't be used. This is imperfect, also, as
the watermark is either small enough to be cropped out or large enough
that it obscures the whole image (thus making it difficult for a
potential buyer to really see what they're buying).

With pdfs, the issue is even trickier. The web is an open platform,
designed for sharing, and so are pdfs. It's only recently that
browsers can display a pdf at all, and even then it's really an add-on
feature. Thus, pdfs are designed to be downloaded. There's no way to
stop people from downloading a file if you still want them to be able
to view it in a web browser. At least, no simple way. Maybe you can do
something with Flash, but quite honestly, that would likely be so
cumbersome for your visitors that I'd wonder why you bother.

If you need to make this material available, but don't want people to
have a local copy, then you need something other than a web+browser
solution. You need a password-protected database (possibly that runs
on the internet) that allows people access to view images of the pages
(not pdfs!) but blocks people from grabbing them in any way. (Be
warned, however, that if they can view it onscreen, they can still
take a screenshot of the page as described in the photography section
above.) These type of solutions are likely custom, and expensive, and
complicated to keep running.

So, finding the right solution depends on exactly what you're trying
to accomplish. What's the goal? Blocking pdf downloads isn't a goal --
I mean, are you trying to work within some out-dated restriction from
a board of directors? Or a donation restriction? The precise nature of
the problem will show you what options you have. Well, that and your
budget.

-- 
Margaret
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