A "PDF Portfolio" is a specific kind of combined PDF file where the merged originals are attached to the main PDF. Is this what you're referring to?
If not (as I suspect), and you just mean a regular PDF file then there are two options:
- If you're on a Windows machine then you can use the Adobe PDF printer and the Multiple option in the Print dialog to generate a new file with multiple pages on each sheet.
- If you're on a Mac machine then you don't have the Adobe PDF printer and your only option is to do it using a script, like this one I've developed and which you can purchase from here: Custom-made Adobe Scripts: Acrobat -- Print Multiple Pages Per Sheet