PDF issue on iPad

[PaulBeijer]

Hello,
I'm new to this forum, but thought I might try it to get advice on a viewing issue I have. Haven't been able to find this issue in an existing thread (but have only had the stamina to search and browse a limited selection - pls advise if answer is already out there).
I use Calibre to manage my books on an iPad 1 (the iBooks app). Many are native pdf files of scanned books with pages containing a mixture of text, photos and illustrations. These go into iBook's pdf collection without any conversion. While most of these pdf files work all right, a limited number are causing problems. When viewed in Adobe Acrobat 9 Pro or Adobe Reader 9, or Calibre's own viewer, they look just fine. But on the iPad itself, the text has disappeared or appears only as a shadow or with extremely low resolution (varies from page to page in same file), while all images and graphics are reproduced accurately (including shading in text boxes - just no text in them !). I suspect there might be some way of using Adobe Acrobat 9 Pro to reconfigure/re-format these files so they become readable, but am not particularly knowledgeable about the pdf format myself. Is there such a fix ? Thanks in advance for any advice you may have.