The Outlook signatures are stored in the %APPDATA%\Microsoft\Signatures\ folder in text, RTF and HTML formats. In case of multiple signatures, the default can be found through Word automation. Tested in Outlook 2003 and later.

Detailed info on Acrobat automation can be found in Interapplication Communication API Reference from Acrobat 8.1 SDK or Acrobat 9.0 SDK at http://www.adobe.com/devnet/acrobat/?navID=downloads. Free registration may be required.

Document properties, both built-in and custom, are a Microsoft Office-wide feature. The Word 2003 and Microsoft Office Excel expose document properties and provide the same way to work with them. These properties provide a powerful way to add and track specific information for a document.

The Outlook model doesn't provide access to the Label property, but it can be retrieved/changed via CDO 1.21.
Requires Outlook 2002 or later. Tested with Outlook 2003 only.
Based on http://www.outlookcode.com/codedetail.aspx?id=139

Code is based on Building Applications with Microsoft Outlook Version 2002