In the search box, you only have to type letters. In your PDF folder, if you type A, you will get all the files with a in them. If you type Ax you will get those files. The more letters you type, the fewer files will be available from the search.
I am not sure why you are using name:
The search is also affected by the Indexing options.
If you can find a file, as an example, I have on my system, maybe I could demonstrate some additional options. If you open the Help and Support and try entering "Searching in Windows: frequently asked questions" in the search box, it might also help.
The filters that might be available could be dependent on the type of folder, or your view settings.