Can I download this document as a single file?

Yes and no. You can download the snapshot backup of the pbwiki site as a single file. Then you can use any ZIP utility to extract the contents.

http://pnlpfaq.pbwiki.com/f/pnlpfaq.zip

The ZIP file contains two sets of files: the HTML output and the raw wikitext.

Unfortunately, the HTML backups at this time are not linked automatically to each other, so you have to find the correct HTML file yourself. Clicking on the links from the FrontPage.html won't work. Additionally, the CSS attributes are lost.

The backup is not done periodically.