A quick mystery solved on a customer intranet today.
You tag some PDF files, alongside other content in SharePoint 2010.
Then, you search for content for that tag, either by clicking a tag or searching for a specific tag.
The results do not include the PDF files.
Luckily, it’s just that the SharePoint search does not index PDF files by default, skipping them completely.
You have to install Adobe IFilter for PDF files on your SharePoint server(s) and configure some other things in SharePoint. After that, you have to run a full crawl again to rebuild the search index.
You can find detailed instructions on how to enable PDF files to be indexed in this Microsoft Support article.