This could be because you are using a PDF with different dimensions than what we currently support.
HelloWorks Builder Beta currently supports PDF with dimensions - 8.5X11 inches (US Letter).
If you have a different PDF dimension like 8.5X14 (Legal), Here are few steps to help you convert it to Letter format:
On a Mac:
1. Open your Legal sized PDF document on Preview
2. Click on File -> Select "Export as PDF"
3. Click on "Show Details" and verify the paper size is "US Letter"
4. Click "Save" to complete the conversion.
1. Open your Legal sized PDF document on Adobe Acrobat or Reader application
2. Click "Printer," then "Print" and "Properties."
3. Click "Advanced" and check the box next to "Letter."
4. Click "OK." This will change the legal-size PDF document to a letter size