A quote from Google:
How can I influence the title shown in search results for my PDF document?
A: We use two main elements to determine the title shown: the title metadata within the file, and the anchor text of links pointing to the PDF file. To give our algorithms a strong signal about the proper title to use, we recommend updating both.
For PDF optimization, PDF Metadata can be used. Searching for PDF metadata editor, you find two applications PDF INFO and BeCyPDFMetaEdit. Use them to edit PDF file's name, author, title, keywords, and so on.