In the error message:
Click on the link to "Design Tab", this takes you to the "Design Tab" and sets the "View" mode to "Page View":
There is a pop up by the "View" selector at the bottom right that lets you know you are now in Page View. Page View is for designing your form for PDF, Web View is for designing it for the HTML/web form.
In Page View the contents that do not fit on the page are highlighted with a red overlay:
There you can adjust the contents to fit. This FAQ on designing your form for PDF will give you some pointers on Page View: