Not sure if this is in the right section, as I don't know how to begin doing it!

I have several pdfs on my site which I sell using Paypal. I am currently emailing them out on payment, but it's not a very efficient system, so I'm going to use the redirect feature on Paypal to send the buyer to a page with a link to the pdf.

I then want to have a button or link for the buyer to click to download the pdf to their own computer, rather than have the pdf opening up in the browser.

I guess html won't do this? PHP? Javascript? Anybody got a simple script/link where I can copy and paste some simple code?

Put it in a zip file and point to it.

If zipped html will work fine.

For security reasons you will need to change the return URL every now and then so people can not share the link with friends.

To comply with PP standards you must have the landing page and can not specify the download link as the return URL. You landing page should contain something like:
You have paid at PayPal and this is your download page.

The actual text is on the PayPal site.

Yep, just put it in a zip folder and then upload it to your host space. Then just point to it. If somone clicks the link it should download automatically.

Thanks! have done as instructed - just got to sit back and wait for someone to buy now

I've already used a Paypal landing page so I know the orders - but thanks anyway

Is it pretty universal that people have unzipping software? I put a link to WinZip on the page just in case.

Doing a zipped file is nice and simple so I'll probably stick with it, but out of interest, how would it be done without zipping a pdf? I know I have clicked on "download pdf" links before which get sent directly to my firefox downloads. How is that done - ie what kind of code?

Windows will automatically unzip anything (though not zip it), so that is no problem. Don't know about Mac, but I'd be shocked if it didn't.

Windows will automatically unzip anything

It will handle all zip files. It won't do tar, gz or rar files. For those you need winrar.

and only in WinXP and above