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Do you have any errors when you try to upload the PDF's, does this just happen with the PDF's or any file you are trying to upload?
fyi, file uploads are a not as cut an clear as to setting your phpini to 800meg (which tbh this is really far to big for a file form upload). I would upload files those sizes using FTP protocols instead.
Another issue you might encounter is that no matter how large you set php ini to, if the form size doesn't allow for larger than 1 meg, it just isn't going to happen. Check HTTP_MAX_SIZE in the form, it could be limiting uploads.
Turn of php debugging in XOOPS and try uploading again and see if you get any errors during the upload process.