It depends on the security settings the creator of the PDF has set, they may allow you to edit or they may not?? That is one of the biggest selling factors for PDF

Look at a company that creates a full QMS (Quality Management System) with approx. 100 health and safety documents. They sell this service for thousands of pounds. In the past it was really easy (using find and replace) to alter the word documents for a different company and sell the documents onwards.....now using PDF and the security settings their intelectual property (if you like) remains theirs!! As you simply can not edit it.

Obviously there are ways around these security settings (as with anything thesedays) but genrally they are secure!!

As for the shrinking element. If you have a word document at 2mb the best you could hope to zip it is maybe to 1mb or 1.3mb but as PDF it more like 0.2mb - 0.5mb