Once you post something, you have 15 minutes to edit your post. After that, you can't - you can only add another post correcting something you said previously.
My question is: what is the purpose of editing a post later on your computer? And do you only want to save your own posts? If so, then simply copy and paste your post into a TextEdit document - you can edit/change it there much easier than in a PDF.
FWIW, you can find all your posts on your own page - I have mine set to see the last 100 posts.
If you also want to download other people's posts, the issue here is the fact that the contents of these forums/posts are subject to these terms and things become a bit more difficult:
License to Use Submission. By sending a Submission to the Site you agree to grant Apple a perpetual, royalty-free, non-exclusive worldwide right and license to use, reproduce, modify, adapt, publish, translate, create derivative works from, distribute, perform, play, make available to the public, and exercise all copyright and publicity rights with respect to the Submission. If you do not want to grant to Apple the rights set out above, do not send your Submission to the Site.
When someone posts here, they automatically have to give Apple the right to use their content due to above terms; however, they retain ownership of their content.