If you want to open a Pages, Keynote, or Numbers document in another app like Microsoft Word, you must first use Pages, Keynote, or Numbers to convert the document to the appropriate format.
Open the File
Open your file that will be exported to Word (Pages), Excel (Numbers), or PowerPoint (Keynote)
Choose Export To
Choose File > Export To, then select the format (Word for example). You might have either menu displayed below depending on your Mac and Pages, Keynote, or Numbers version
Save As, Finish
After the export steps above, you might also be prompted to Save→As
Click Next, then enter a name for your file, then choose where to save it
Click Export when finished.