How do I change the view of my document from left to centre?
If you are finding that your document is off to the right when viewing this may be due to the 'Style' of the document being set to a 'Book' style. You can change this to the 'Presentation' style within the 'Manage' screen of the document under 'Style' tab.

See further info here -> Applying a Style to Your Document
If you have a custom style, you can set your document to display as a single page (one page) instead so the View is centered, to do this, you’ll want to adjust the Document View settings within your Style options:
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Go to Documents in the main menu.
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Select Settings, then Styles.
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Choose to edit your existing Style or create a new one.
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Under the Document View tab, set your pages to display in single-page mode rather than double-page (book style).

This will present your document as a one pager to your readers. If your layout is in Presentation (landscape), single-page mode is especially suitable for a cleaner, more streamlined look.
You can find more information on Style settings here -> Creating a Style/Style Settings