I love filling out forms--they're like tests where I know all the answers. Starting in Mountain Lion, entering data in PDF forms got even easier; Preview can now automatically detect where form fields are, so blanks automatically become text fields where you can type and checkboxes can simply be clicked.
Great as that is, when I sent those filled-out PDF forms to some of my colleagues, I found they'd often reply that the form I'd sent them was empty. And yet, when I opened the same document on my own computer, the data I entered was still there. What gives?
Well, after a little investigation I determined that the problem seems to be--surprise!--that those folks are Windows users. For some reason, the software they use to view PDFs (likely Adobe Acrobat) doesn't recognize whatever Preview is doing to support entering that form data, so those PDFs simply register as empty.
As it turns out, the fix is simple, but a little silly: After you've filled out a PDF form in Preview, choose Print from the File menu and then use the dialog box's PDF option to save it to a PDF.
While it might seem like you could accomplish the same thing using the Export As command in the File menu, it doesn't work in the same way--the Print option actually takes the data you've entered into the form and turns it into part of the PDF (if you prefer the technical term, it rasterizes it). On the downside, that means that you can't edit that text any more, but it also means that your PC-using friends and colleagues will actually be able to see the form data, because it's no longer editable text, but a picture of that text.
If you're sending the filled-out PDF form to a fellow Mac user, you don't have to go through this whole rigmarole. However, saving to a PDF does have an additional benefit in that case: Since the form data gets converted into an image, saving to PDF means the recipient cannot easily edit the data that you've already entered.