On my MacBook Pro running Sierra I have this problem: when I look for a text string in a MS Word ".docx" file the search function does not find it although it can be found all right when I save the same file as ".doc". What can I do to also find strings in .docx files?
There is a free Appstore app named Easyfind which enables you to set certain search parameters. When you open Easyfind you get the pane shown below.
Click on the Settings button in the top left corner and it opens the small additional pane on the left. You can set Easyfind to search files or folders or both and then using the small pane on the left you can click on one of the groups and be sure to add docx. After that just include your search phrase and hit the return key.