After I create a document in Pages, and then export it to a folder in Finder as a PDF, how can I later edit that document? When it comes up in Adobe Reader it won't allow changes.
Once you export your document to PDF you will not be able to edit it in the future. Creating a PDF document that can be edited involves using an application which has the ability to create PDFs with the feature to add security options. You should keep a backup of the original document so you can modify it as you want and then export it again to the PDF format or use Acrobat X in order to create PDF files and modify them for later use but the suggestion with the original document is a better approach than using Acrobat X.