PDFs are usually viewed in programs without PDF-editing tools. Given that fact, creating a PDF with editable fields lets you create forms that are resistant to tampering or inadvertent alteration ...

To modify a flat form -- one without editable fields -- you need to use the Add Text tool. If you have Acrobat, you can select this tool in any PDF through the "Sign" button, but you can only add ...

If you need to save a filled-out PDF form so the content is no longer editable, you can do so with Preview in OS X.