MobiOffice lets you protect your document on a file level, allowing no one to view the file unless they have the password.
Password-protect a Document
1. Open a file in MobiDocs on your iOS device.
2. Open the document. Tap the menu in the top right. Tap Protect.
3. Type and retype the password.
4. Tap OK.
5. Save or Save as the Document.
To Remove a password
1. Open the document. Tap the menu in the top right. Tap Unprotect.