More tests about Kindle 2 Blank Page:
According to this thread, Kindle 2 display area is 520x622. 622 is not multiple of 8, so I thought that might be the problem.
Last night, I tried PDFRead to create .prc files with size 520x616, 520x618, 520x622, 520x624. All had blank pages problems except the 520x616. I was excited and thought I found the reason.
But, today, I tried the 520x616 again, the blank page problem shows up too.
I swear I saw it work fine yesterday.