Leo says that PDF Portfolios are multiple files that are put together into one master PDF file. Adobe did this to get a handle on the PDF format, which has pretty much gone open source. PDF stands for "portable document format" and it's essentially an image of a document that preserves formatting across a multiple formats and operating systems.
Now everyone uses PDF. In order to preserve Adobe's business, however, they've started adding extensions to them, like Portfolio. Leo advises avoiding them because it's likely that his third party reader won't be able to read it. He can use standard PDF and even join multiple PDFs into a master PDF file without having this 'Portfolio'.