I change the margins from the default of 79 to 50 to save pages when saving. My issue is I have to change it everytime I make a document.
Documents are always based on some template, be it the default template.
A template contains definitions and settings for all styles, including page styles.
To have your favourite settings automatically applied when you create a document, make a template: a template is nothing more than an ordinary document saved with a specific extension.
When all your styles (or just those you use) have been modified to your taste, use File>Templates>Save as Template to save it in the My Templates category.
After saving it, File>Templates>Manage Templates,right- click on your new template and Set As Default.
Note: you did not mention you OS name. Under Windows, creating a new document from the file browser (or whatever its name) will not reference your custom template, even if it is the default. Consequently, open first Writer and File>New>Templates to start a new document based on your template.
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