A family of Microsoft word processing software products for creating web, email, and print documents.
I see what you are referring to. The top page is slightly narrower than bottom page.
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I would guess that Top one is "A4" and the bottom is Letter. Depending on selected printer, ie PDF, it could have both printer types. https://en.wikipedia.org/wiki/Paper_size
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Paper size is a document and "section" attribute. Check to see if you have any unwanted / unneeded section breaks.
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Try selecting the whole document and applying your preferred paper size.
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Specifically what other "formatting" issues do you have?
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I haven't worked with the "Protect/Lock" feature so you'll have to do some more digging, but you MAY be able to "Protect" your document from the types of changes causing you problems.
Here are a couple of articles
How to create a userproof locked editable form in Microsoft Word
https://vapromag.co.uk/how-to-create-a-userproof-locked-editable-form-in-microsoft-word/
Creating forms with Microsoft Word is useful and easy, but the challenge comes in when you decide to create fillable forms with options that you can send to colleagues or clients and have them fill out the form digitally. Whether you need a form for gathering information or if you are trying to create a survey to test out user response to software or a new product, Microsoft Word can assist in this process. It’s a slightly more technical job and requires access to the Developer tab in Microsoft Word, but is more than easily achievable with not too much hassle!
Protecting Document Areas from Global Replacements
https://wordribbon.tips.net/T010271_Protecting_Document_Areas_from_Global_Replacements.html
You may have boilerplate text that you need to include in your document, and it would be detrimental to accidently change anything in that text. The solution is to protect the unchangeable text, as described in this tip.