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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..