File with .ASHX extention is an ASP.NET Web handler file.

ASP.NET webserver will process the ASP.NET hanlder to a web browser's request.
It was first introduced in Jan 2002 with .NET Framework version 1.0 and to be the successor for Microsoft ASP - Active Server Pages technology. Active Server Pages were in the backend for Microsoft IIS server-side execution environment that enables running ASP scripts.

Once in awhile you might received a .ASHX file as an attachement or you have downloaded a power point presentations from the web and the file extention is not a .pptx but a .ASHX file.

What you need to do is rename the file extention from .ASHX to .PDF and open the file with a Adobe Reader or PDF viewer.
Sometimes you can fix this by making sure that the web browser have a Adobe plug-in installed to view and open PDF file directly in the browser.

Other Programs that open a .ASHX files:

Mac OS - TextMate & Web brower like Safari or any other

Windows OS - Microsoft Visual Web Developer, Microsoft Visual Studio 2010, any Web browser and any text editor.