Styles are at the heart of every Word document, regardless of whether you're using them correctly.

When working with different languages, one might have one paragraph Style for, say, the English text, and another for the Hindi text. Where both languages are required in the same paragraph, one might type these up in separate paragraphs, then apply a Style separator to join them visually; alternatively, one might supplement the paragraph Styles with character Styles (e.g. one name 'English', the other 'Hindi'), using the paragraph Style for the dominate language in a given paragraph and the character Style for the other language.

If you upload two documents, one containing content from your 'first' or 'second' file, and another containing the problematic content from your 'third' file, it may be possible someone here to remedy the issue via Style redefinition or a different approach to combining the content.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]