When you upload a PDF the file name automatically translates to the document name. It's easy to change this name later.
Go to your Docs in the main navigation > Document library and then select the document with the name to change.
The title of the PDF can be found at the top of the page, click on it and it will auto-magically turn into a text input. Write in the new title and click the Save title button to the right-hand side. That's it, your document title is changed.