FM ran out of memory

FrameMaker 12 on Windows 7 platform (new FrameMaker 12 user but have been using FrameMaker since 1996): I have a large manual comprised of multiple files. When I save the book as a pdf file, I get  "FM ran out of memory" response, and then "Adobe FrameMaker 12 has stopped working" message. It creates a pdf file but won't let  make alterations to the pdf file and then save it using the same name.