It depends on the analyzer used for the fields.
Probably foo.pdf has been indexed as foo, pdf.
If you don't understand, could you provide a full recreation script as described in About the Elasticsearch category. It will help to better understand what you are doing. Please, try to keep the example as simple as possible.
A full reproduction script will help readers to understand, reproduce and if needed fix your problem. It will also most likely help to get a faster answer.