When you login into your paid account, can you create more than 1 form? This just to verify if that account is a paid account. If the accound is not a paid account, you would not able to create multiple forms. And, after you verify the account is a paid account, and still run into this issue? Please let me know.
One more thing to check, just a thought - is the form you are trying to save as PDF a form that you are the author of and not a form that was shared with you? The error dialog would be specific in the case of a form shared with you by a "Free" user stating the form author must have a paid account...