Saving modified pdf files using Preview

[Barefoot]

I received an email with a pdf file that I had to complete and send back. I opened the document with Preview, made the changes to complete the form, and saved it ("saved as" because the program wouldn't let me just "save" it. A box popped up saying that changes may affect the original format and that I had to "save as" to get a copy of the file). I opened it again just to make sure, and all the changes were there. I sent it via gmail, and a few hours later the original sender tells me that the form was empty. I checked, and it was empty, even though the name of the file was the one I gave it. I tried again with the same result. Using a PC I had no problems, so I am guessing that the issue is with Preview. Does anyone know what I did wrong? Sorry if this stupid, I've never used Preview to modify files before.

 

[GGJstudios]

Adobe Reader for Macintosh

 

[Barefoot]

Thank you! I had the program, but I didn't think of using it.