One of Microsoft’s patches messed up the Outlook Search. The patch to fix it came out on June 27, 2017.

Depending on what version of windows you have, you need to make sure you have a certain patch to fix the Search problem. See the following link. (after opening the link scroll down and open Issue #5)

https://support.office.com/en-us/article/Outlook-known-issues-in-the-June-2017-security-updates-3f6dbffd-8505-492d-b19f-b3b89369ed9b?ui=en-US&rs=en-US&ad=US

also this link lead me to the above link

https://partnersupport.microsoft.com/en-us/par_clientsol/forum/par_outlook/office-365-outlook-error/f2164c46-c346-431d-8f88-5a0078c338ab?auth=1

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Another reason that the search is not working is because of a PDF filter. check this out

https://support.microsoft.com/en-us/help/2925203/incomplete-search-results-may-occur-in-outlook-when-a-pdf-ifilter-is-i

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https://support.microsoft.com/en-us/help/2769651/outlook-search-returns-no-matches-found

If in general you have a problem with Outlook search read the following link.

https://www.msoutlook.info/question/47

The key is:

1. Open Oultook
2. Open Control Panel / Indexing Options
3. Click Modify
4. Uncheck the Outlook profile that is is having the problem.
5. Click OK
6. Close Outlook and make sure it is closed
7. Open Outlook
8. Go back to Indexing Options
9. Click Modify
10. Put the check back on the Outlook profile
11. Click OK
12. Open Outlook