You could cache the pdf files as they're generated and produce a cropped and reduced size (and watermarked!) PNG file from the images using ImageMagick (or similar) on your server. Dump those PNG to a folder occassionally (eg cron job) and use a "coverflow" style generator (eg http://www.imageflow.nl/ ) to add some panache to the page and show what type of worksheets people are using. If you do I'll bet you get more clicks via the coverflow links than any other route.
Edit: looking again I'd really make an effort to add both a link and a graphical watermark to the free printouts. One upgrade you can offer is to remove the watermark. Another is to add the school name and logo in place of your own. The name/logo (no logo yet?) is _free_ marketing ... headteacher see the worksheet, tells superintendent, informs all teachers in the region ..., etc..
And the idea behind the HTML was the same. It doesn't have to be pretty, just some kind of preview. Giving sample images might remove the problem, but when you're trying to figure out how hard the worksheet is going to be by tweaking the configuration, it'd be nice to just see an instant update of what the problems will look like. You could just show a simple plain text representation of a representative problem based on the current input and that would be very helpful in my mind.
Edit: looking again I'd really make an effort to add both a link and a graphical watermark to the free printouts. One upgrade you can offer is to remove the watermark. Another is to add the school name and logo in place of your own. The name/logo (no logo yet?) is _free_ marketing ... headteacher see the worksheet, tells superintendent, informs all teachers in the region ..., etc..