To address your questions in order:
1. There are probably other programs on PCs that can open pdfs. On Macs Preview is the default app for pdfs, unless the user has changed it. If the user has installed Reader, Reader often "takes over" that file association.
2. Offering the form in Word as an option isn't a bad idea -- people who have Word will be able to use it. (It sounds like you understand that offering it only in Word would be problematic.) It's possible to set a Word document as read-only, so that people couldn't change it.
3. Many many Mac users right-click. I use a 5-button mouse with scroll wheel on my Mac, and I right-click all the time. And middle-click, and scroll, etc. PDFs are not a problem for Mac users (no more so than for PC users, anyway), and there's no need to differentiate between the two. You might include in your right-click instructions that Mac users with a one-button mouse need to "control-click" or "click-and-hold" to download the file. (Control-click and click-and-hold both emulate a right-click.)
With a regular click on a link to a pdf file, whether or not it opens in the user's browser, opens in Reader as a separate app, or downloads to their computer is entirely up to how the user has their browser configured. You could zip it to force a download, but then your users would have to find it and unzip it before they could open it -- one more step, and one more place for user confusion to creep in. You can also user server-side scripting to add headers to force the pdf to download rather than open -- but then you're interfering with the people who want it to open in their browser window.
You're on the right track -- tell them it's a pdf, tell them how big it is, tell them how to right-click and choose where to save it. Offering to snail-mail it to them is a nice bonus. But no matter what you do, you'll get users who don't know how to download it, or they download it but don't realize it (and they keep clicking and clicking), or they know they downloaded it but they don't know where to find it.