Nikee. You can't save your work as a pdf and still be able to open it at a later date. pdf is an export for a save function. You will always have your original fdr file to work on.
If you have FD 7 would need to update your software to the newest update that fixes any problems with having HUGE pdf files, but if you have an older version like 6 (5 and below do not have pdf features), you will need to buy an upgrade since I don't think FD 6 has a fix for the pdf export, but email FD and they can tell you better.
go here :
http://www.finaldraft.com/support/email-support.php
FD is constantly looking at bugs (as reported by users and beta testers like myself) and releases patches and fixes every few months. They are very proactive in making sure you have a good program to work with, but with so many platforms, configurations and such, bugs will always pop in from time to time.
Now Xhouse. I'm sure you have a legitimate copy and this is not meant for you, but FOR THOSE who know who they are...
If you have a FD (or any software) bootleg, you don't get any support, updates or fixes. The price you pay for not paying.
I get complaints from many people who say their FD crashes or loses info or does some stuff that is not acceptable. When I ask and they tell me it's a bootleg, I don't have to try hard to explain why they are having problems.