How do I complete MS Word document on my iPad using Pages?

Once you open a Word document in Pages, the current and translated document is a Pages document and the Word document does not remain open for editing. The Pages document translation process excludes those Word features that have no counterpart in Pages. That means no functional form fields or signature fields will work once opened in Pages.

We have recommended for years that if you are receiving Word documents from Word users, then you should be using Word yourself to natively support the document's Word features and its editing. Pages is not a Word clone and may cause you considerable lost productivity, if not a reprimand when turning around documents sent to you by Word users who use form fields, signature fields, and other Word-specific features.