I have enjoyed using the excellent post2pdf-converter plugin. And I also use the tcpdf library which has been very helpful and very frustrating from time to time. On WP 3.34, I have observed that posts with PNG images when saving as a pdf using this plugin causes near 100% CPU until the process times out. This also happed on the previous versions of WP 3.3x.
I disabled all other plugins but still I get always the runaway CPU. The tcpdf program, while a wonderful facility has no error return values and I think that cause a lot of us problems tatcking down errors.
If anyone has a solution for this PNG specific problem, I will be happy, happy!