Then the PDF-ShellTools registration code is missing some corner case.
If you have the time, search the Windows registry for this key name {E3FD95C1-D5B1-4929-B0EC-C156318EDD8F}, and take note, in a text editor, of all the key paths of the found ones.
Use the ShellExView tool to disable the PDFST context menu extension, and repeat the above registry search.
Now use the PDFST manager.exe to enable the extension, and repeat the above registry search.
Notice any change?
Is the context menu item still showing, or you need to use the ShellExView tool again?