It is important to define a document title as it is the first thing a screen reader will read. Providing a document title allows the user to locate and identify the document.
If the document title is not defined, the screen reader will read the filename which can be something out of context.
Also, it is important that the defined title is meaningful and reflects the content of the document. This will help the user identifying the document and understanding what the document is about.
A document's title can be found under the Info page.