In the Page Tab you will find the option for Margin. This option opens a dropdown providing common margin sizes. You can select one of these to update the margin.
You can also create custom margins for your pages. At the bottom of the dropdown menu, choose Page Margin Setup.
Choose the page range that will receive the new margin size.
Pick the unit of measurement from the dropdown.
You will need to set the measurements for your margin. The number in each box refers to the unit of measurement you selected, update the number in each box to customize your margin.
You can use your custom margin size as the default for new documents you create. Click on Make Default to set this.
*** The ability to set the default is presently unavailable in Soda PDF Online
Click on OK to apply your changes.