Please can you tell which browser are you using, this should happen only if you disabled javascript or try to change the parameter type of the export. If that's neither your case we will trow one of our programmer to the tiger:)
Nice service. Only advice I can give is to tweak the typography a bit for the PDF output: I'd restrict the measure (line lengths) to around sixty characters, and boost the leading (space between lines) to about 1.5 times the line-height. Personally, I'd make the body text a bit smaller too, but the bigger text might be preferred by some.