Printing PDF with A4 & A3

So, just so I understand you correctly, you would like the A4 document to print as either A4 or A3 and vice versa? If so, you'll want to make sure that the Choose paper source by PDF page size is not checked in the print dialog. Then, you need to use the Fit To Printable area command under the Page Scaling dropdown to scale small pages up and large pages down to fit the paper source.

I have assembled a PDF portfolio. Most of the document is A4, but some are A3. I want to know if it's possible to print the portfolio, so that the A4 pages print on A4 and the A3 pages print on A3. Currently the whole thing prints on A4, which is not suitable for the A3 pages, as they have too much information on to fit on A3 and when printed on A4, it becomes too small...

Ok - in each PDF document you can set a default print dialog property. Go to the File > Properties menu and select the Advanced tab. Check the box for Paper Source by Page Size. This does assume that folks have access to both A3 and A4 paper sources.

So if the pdf is a portfolio of PDFs, will it still work?

If you choose to print the file while in a Portfolio (selected PDFs or all PDFs) then unfortunately no. However, if you click the "Open file" button so the PDF is in a new window, then these settings are maintained.