If you tried to download the Citizenship form N-400, you would notice that it does not opon. It shows wrong file size (106kb) and it says you need pdf to open this file.
The error appears because of security reason. This particular pdf file has high security features so that it can not be edited and saved. Mac OSX Snow Leopard, Lion, Mountain Lion all have default pdf writers. That means you do not need to download pdf reader or writer. The OSX itself allows to save any file as pdf or edit a pdf. N-400 form cannot be opened on a system which has pdf writer installed on it. There it will not open on mac.
You will run into similar problem on Windows if your installed any PDF writer such as PrimoPDF which is basically a driver to write print a document as pdf file. You must uninstall any pdf writer before you could open N-400 form.
You can use XPS reader on Windows to save you your pdf file though. Simply open the file as pdf, fill it out and then issue print command. Make sure you select XPS document when printing. The file will be save with data. Of course you can only open this document on a windows plateform where XPS is installed. XPS is installed on all Windows 7 (and Vista I guess) PCs. This solution works pretty well.