1. From the "File/Print.." menu
Go "Page Setup" tab and click the "Page size" button, and choose the "Manage Custom Sizes...". There you can set the paper size with the margins.
For example, setting 8.26x11.7 inch paper with 0.4 inch margins makes A4 with 10mm margins. The setting can be named "A4-margin10mm".
2. Tweak the setting file
In the ~/.config/pluma directory, the file "pluma-page-setup" is the one for the page size and margin setting file for printing. It can be written like the following.