Hi. I don't know specifically about .zip submissions, but I always upload
the raw .rtf file without compression. I have never had a problem with an
invalid file this way. I know that sometimes it chokes on files with
spaces in the names, but I have never had a problem with that either. I
would suggest not using zip at all. I don't know what causes invalid .rtf
files, but I would try opening them in Word. If Word crashes, it's
probably corrupted somehow. If Word opens it but the step 2 page rejects
it, you could try adding and deleting a character and saving, or you could
save as .doc, open the .doc, and save again as .rtf.
