When I found out that Goodreader crashes or doesn't even open powerpoint files larger than 30mb, i decided to buy Document to Go. After reading many reviews it seemed to be the logical choice. Unfortunately pptx files are extremely slow. If d2g opens them, there is a lag up to two minutes per slide. Simply amazing. Fortunately, my samsung galaxy s2 always rescues the overpriced ipad. Doesn't matter how large the pptx file is, my android phone is able to download the file from anywhere (internet or cloud), stores it on a mutual location which can be accessed by any app and opens the pptx file in its stock app without any hiccup.

Just wondering if you run into the same problem with an earlier version of PowerPoint files, i.e. .ppt rather than .pptx. My experience with most Office emulation suites, including Documents To Go, is that they tend to work better with older versions of the files created by Office.

Just for an alternative, you could look into QuickOffice HD. Ive been using that for a few eeeks and so far, so good. It has a slick interface and good performance from what I can tell. Overall, I wasnt so sure about the Office experience on the iPad but Im quite pleased with my current solution.

I think it's Apple's own limitation, cause it's own stock viewer doesn't open large files either.