When the PDF Icon is missing, but the pdf can still be opened from HowNow, the .ico file needs replacing in the registry settings.

This solution specifically relates to clients using PDFXchange which doesn't have it's own icon. The process is replicable with other PDF solutions.

Essentially you are copying the Adobe Acrobat icon and placing it in the PDFXchange folder.
>Regedit

>HKEY_CLASSES_ROOT

>AcroExch.Document.DC

>DefaultIcon

>copy the default icon file location

Navigate to:

>PDF-XChangeviewer.1

>DefaultIcon

>and paste the icon file in as the default.

More information, including registry key names for various programs.
​Default PDF Viewer Not Opening Correctly