If you export a PDF files into JPEG, sync to iPad with iTunes, the pictures are not shown in the correct order. There is no setting to change the sort order to filename instead. (by creation time?)
1-2-3-4-5-6-7-8-9-10 becomes 5-4-3-2-1-9-8-7-6-10
a-b-c-d-e-f-g-h-i-j becomes c-b-a-g-f-e-d-j-i-h
An alternative if you want to show album like that is to use Adobe Acrobat PDF portfolio.
