I have encountered a strange behavior in Preview (10.5.1) with PDF forms. If you open a PDF form, fill it out and try to save it, you get a message saying "If you overwrite the original file some PDF attributes may be lost. Choose Save As . . . to save a copy instead." The Save button is changed to Save As.
But when you hit the Save As button and choose a different file name, all of the information you have entered into the PDF form is lost, and the form reverts back to a blank form. Strange.
The only workaround I have found is to print the completed form as a PDF file from the print menu instead of using the Save As button. This creates a a PDF file that retains the information you have entered into the form. You do lose the option of re-editing the information you entered into the form, but you can add information to the now filled-in file using something like FormMate or an equivalent application.
Hope this might save someone else some time.