Check the PDF settings and upload the PDF again.
If you see lines or colored page sections which are not present in the PDF, the file probably contains too many vector elements or unsupported layout features. Save the PDF as an Acrobat 4 (PDF 1.3) file and upload it again. You can try re-saving your PDF in Preview for Mac or similar software as well as it can remove errors.
If one or more pages are still not converted well, replace your vector graphics with bitmaps (JPG) on these pages, save the PDF as an Acrobat 4 (PDF 1.3) file and upload it again.
If the problems still occurs, contact us by email to inform us about the problem and send us the non-flattened file (minimum PDF 1.4) via a file sharing program like https://www.wetransfer.com. Clearly mention in your email the page numbers on which the problem occurs and add a direct link to these pages in your email.