Word to PDF failing when digitally signing

Hi,
 
I'm currently using Acrobat 6.0.6. I'm converting a large Microsoft Word document to a PDF file by printing to Adobe PDF or Coverting to PDF using Word. After I convert, I need to digitally sign the document.
 
This document is about 100 pages in Word, and 5MB in file size. When converted to PDF, the filesize goes up to 18MB (this seemed odd to me). The document has many images and different style text, etc.
 
When I try to sign the PDF file, it cannot complete the task, saying that it ran out of memory. In fact, I can't alter the PDF at all without it saying this. Any help would be appreciated!