PNG to PDF?

So I got some homework and it is some kind of book. I made it page by page with Photoshop (should have used Corel or Illustrator I know)

Now I've the pages separated one by one in a PNG extension, and I want to make a PDF out of it. I've been searching for programs but they're all free trials and leave watermarks so not using them.

Anyone got any other ideas?

If you have a nice scanner you can scan it back to your computer as a PDF.

Have you tried http://www.cutepdf.com/ ?