You need to adjust the height and width of the certificate so that it fits the print page.
Please note that wplms uses a script that takes screenshot of the content of certificate which is rendered in Html and converts it into an image.
For your issue please try reducing the height of the certificate template and also apply (margin-top:-50px) to your cetificate image to remove space form the top.
There is limitation in the certificates you have to adjust a bit for this ,just focus on the certificate not the white background in print preview and in popup.We are trying to find a better script for the certificates because existing scripts have many issues .We will update the certificate generation module to encounter all these problems .But till then you have to adjust with the script limitations.