metadata extraction from PDF is already contained as part of the standard platform. Usually, there are no add-ons required for this unless you have a very specific requirement. What are you trying to do? Where do you think the default functionality of Alfresco provides too little support?
author is one example of metadata that should already be extracted automatically when you upload a PDF via Share. You can also trigger metadata extraction by starting the action "extract-metadata" (ContentMetadataExtracter) via a script / rule. "Modified By" (cm:modified) isn't a property that should be extracted - this should only be maintained / managed by the system. You could of course define a custom property and map the extracted value of any "Modified By" document header via the PdfBoxMetadataExtracter.properties mapping configuration. Please also have a look at the <a href="http://wiki.alfresco.com/wiki/Metadata_Extraction">wiki article about metadata extraction</a>.