I thought that I had the same issue, but I remembered that I had done it before (and the Google Chrome Book is the only laptop that I've used). The issue is that we are searching for "PDF." to be under "Save As" BUT IT's NOT. Here's What You Do:Open Google Docs> click on doc to convert> make sure MODE (right corner near top) is switched to "Editing" ( not viewing,etc)>click FILE> find "Download As"> click it and choose ".PDF" option.
So there you go!!! I hope it works for you!