The page Stefan referred you to is the best starting place. Turning on display of non-printing formatting characters is key.

Scanned or converted documents are always going to have formatting anomalies. Among those anomalies are frequently section breaks used to change margins instead of changing paragraph indents. In addition, if these were scanned, the text is from an OCR function and will need very careful proofing, probably by more than one reader who understands the jargon being used.

Generally, if I want to incorporate scanned or converted documents into an omnibus document, I will save the separate documents as plain text files (.txt) and then open those in Notepad, copy their contents, and paste into Word. Then I format the Word document the way I want, using Styles.

Indeed, as Charles points out, converting from other file formats, or combining content from several different sources, will always require quite a lot of manual (re-)formatting.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional