Hi @Maree Mills ,
Welcome to Q&A forum!
What's the type of your account(pop, imap or exchange), and what's the sepcific version of your client(File>Office account>About Outlook)? Did this issue occur on any specific PDF file or all PDF file in your Outlook messages.
In order to further confirm if the issue is related to your client, if it's possible, please try to search on your web mail to check if it could work normally there, and if you are working with exchange account in cached mode, it's recommended to switch to online mode to test.
If you could search normally in your web mail, the issue may be related to your Outlook client. According to my tests on Outlook 365 Version 2106 (Build 14131.20278), I could search the message with a PDF attachment that contains the main characters"abcd" like below, due to I haven't installed a 3rd-party PDF perviewer tool, I opened this file in edge:
Please kindly check if my steps have any differences with yours, if the issue still exists after you performed the same tests, considering corrupted search index may cause it, it's recommended to rebuild your search index to check if the issue has any differences after that(Control Panel>Indexing Options>Advanced>Rebuild>OK>Close).
If your issue has any update, please feel free to post back.
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