Word documents in Pages

When you open a Word document in Pages, it is no longer a Word document because it has been (more or less) translated by Pages into a Pages (.pages) document. The original Word document remains unchanged outside of Pages.

Because Pages is not a Word clone, there may be features used in the Word document (e.g. forms, ToC, etc) that have no Pages counterpart and they will be excluded in the translation. If the Word document uses a font reference (as opposed to deliberate font embed), and that font is not physically installed on the Mac, then Pages will attempt font substitution. And then, the layout of the Word document as observed in Word, is not the layout engine used in Pages, so it may not appear the same.

Sometimes, even MS Word balks at opening a Word document it has opened previously, and Pages is no different in that regard. I keep a current version of LibreOffice installed just for such occasions as I have found that opening the Word document in LibreOffice and saving it again as a Word (.docx) document has cured MS Word, and Pages of document rejection. But not always.

The "why" can be almost anything and there may or may not be a fix mentioned above. I have Word documents created by Word 6 in 1997 that Pages wants refuses to open as they are deemed too old.

If Pages refuses to open the Word document, you are likely done unless opening and saving in LibreOffice supercedes Pages refusal to open the document.