Your solution may be configured to display SharePoint document links as DocPop, PDFPop, UnityPop, or AEPop links. Depending on which is used, the document is displayed in a different viewer:
DocPop links are displayed in either the HTML or ActiveX Web Client viewer. For more information on the Web Client viewer, see the following sections.
PDFPop links are displayed as a PDF in a PDF viewer.
UnityPop and AEPop links are displayed in the Unity Client viewer. For more information on the Unity Client viewer, see the Unity Client documentation.