I ran into this same problem when dealing with the PDF export as I am on an Apple and there appears to be issues when working with PDF exports from a Mac.
I tried both the Press presets and downgrading to the Acrobat 4 settings as was suggested here by Alex. It didn’t work for me. (Not picking on Alex, I just didn’t get to the bottom of the issue(s) present.)
The solution that I came up with was to export as full Postscript files and not as PDFs. As my destination was a 300dpi magazine, I ripped the files at 1000dpi so that my rasterized text (when dropped at the printer back to 300dpi) would be crisp and readable. This meant bigger files to upload and an extra step but it worked and ended days of frustration.
I hope you get to the bottom of your issues and if you find the answer to the PDF issue you are encountering, please let me know.
Best regards,
Ron Lindeboom