Click on a .docx icon, and press ⌘i to bring up the Get Info panel. In its Open with: section, change the default application from Word to Pages 7.3, and then click the Change All… button and follow through it.
Pages is neither a Word clone, nor a practical substitute for MS Word when exchanging Word documents with other users that are using Word on Mac and Windows. When you open a Word document in Pages, it will be "best effort" translated into Pages proprietary .pages document format, and the original Word document remains unchanged. All edits will be done to the .pages document, and you must export (another "best effort" translation) to a different Word document name to avoid overwriting the original. Apple makes no assurance that inbound or export translations will be accurate.