Hello
I have a 1000 pages document and want to Save As pages from 350 to 890. Is there any way to save the pages without selecting those pages, paste them and save as it into a new file?
For saving as PDF it is possible, but what about Docx file?

No, Word cannot save just part of a document; you'll have to either delete the unwanted content or copy the wanted content to a new file.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Simplest method (for me) is to make a copy of the file and delete the parts I do not want.

Keep in mind that section formatting is contained is the section break following text. This includes headers, footers, orientation and margins.