There should be free programs that can distill (adobe lingo) multiple images into a single PDF file. I don't think that will be a problem at all, so your best bet is probably to deal with the individual PNG files for now, rather than the multi-page TIFF files. You likely won't find many programs other than FAX software able to properly deal with multi-page TIFF - and then likely only being able to read, not edit.

On a Mac, you could likely use the built-in Automator with a couple of simple actions to put all those images into a PDF.

Here's one program:

http://www.pdfill.com/pdf_tools_free.html

And here's another program:

http://www.Derring.com/swiftpdf/