Been There, Done That
I've already been generating PDF files out of Perl for some time now, just by generating PostScript (OK, so I know a bit of PostScript ..... wouldn't call myself anything like fluent, but I know how to draw simple shapes and stick text on a page) and piping it through pdf2ps. You can even insert images saved as EPS straight into the stream. It doesn't half look impressive (although that's probably more to do with the perception of PDF as being used only for highly "official" documents). As a final touch, I even detected the user's OS and offered a link to an appropriate, non-Adobe PDF reader (healthy competition is a wonderful thing).
Also, if you generate a simple CSV file but give it the extension XLS, Excel will load it without missing a beat (Office doesn't really go for file extensions). You can even provide a second link to download the same file with a CSV extension (for Open Source users), so as not to scare anyone into thinking they absolutely need proprietary software.