Google Docs margins not changing? Check Pages mode

A pageless document has no fixed paper edges, so changing its on-screen text width will not set print margins. Use Pages if the file needs fixed margins for a PDF or printed copy. If you only want shorter lines on your screen, stay in Pageless and choose View → Text width. That preference does not change what collaborators see.

When the document has multiple sections, click in the section you want to change before opening Page setup. In the dialog, check which part of the document the change will apply to, then set the margins and click OK. If a handout needs the same margins throughout, choose the whole document when that option is shown. To find an unexpected boundary, use View → Show section breaks.

Open print preview and compare a page inside the affected section with one outside it. If just one paragraph starts farther in while the rest of the page lines up, check that paragraph’s indentation. Changing the whole page’s margins to fix one paragraph will also move text that already looks right.