Hi Karen,
Yes, you can search for the content present inside the PDF files and as well as contents in Office documents too. For more reference, you can check this article: Default crawled file name extensions and parsed file types in SharePoint Server (This article applies to SharePoint Online also).
If you’re having any problem with Search results, you can check this article: Search results missing in SharePoint Online.
Appreciate your understanding.
Best Regards,
Chitrahaas