Spacing between paragraphs, added as you press Enter, is "Spacing before" or "Spacing after" or both. You can select the whole document by pressing Ctrl+A. On the Page Layout tab, in the Paragraph group, type in zero for "Before" as well as "After."

To change the document defaults: In the Styles pane (Ctrl+Alt+Shift+S), click the Manage Styles button; on the Set Defaults tab, set spacing before and after to zero, make any other changes that you want. Select "New documents based on this template" and click OK.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional