1. Download and update WhatsApp to latest version
2. Install ES File Explorer from Play Store
3. Open ES File Explorer, enable Root Explorer and then navigate todata/data/com.WhatsApp/shared_prefs/com.WhatsApp_preferences.xml with ES Note Editor.
4. You will see the string <string name=documents">PDF</string>
5. Add the extensions doc,txt,docx,pptx,xls besides pdf.
6. Save the editing and force close WhatsApp.
Restart your phone and see how it is working. You are now ready to share any document types (pdf, doc, txt, docx, pptx, xls) with your contacts.