Hi Willem Jan,
When you open the PDF file in EasySIGN as vector data you will see that the PDF file contains many single line curves. In order to create a mask, these lines need to be converted to one single object which can be used in EasySIGN as a mask afterwards. So in this case we recommend to select the lines which need to be used as a mask, and then use Arrange > Combine or CTRL + L
As the PDF files contains many curves this can be time consuming depending on the number of curves that need to be used as a mask. Alternatively, you can check if the PDF file can be exported from the original design software as a combined vector object. This may save time in creating the objects which will be used for masking.
Have an easy day!
Wilbert at EasySIGN