PDF SDK registers itself as ActiveX during the installation. It does not require referencing from VB editor to work, but you can add a reference if you want VB to show property hints.
If properly installed, PDF SDK should be shown in the References.
If it is missing, or the projects shows a error at the run time, please close VB IDE and manually execute the following file As Administrator
c:\Program Files\Bytescout PDF SDK\ActiveX\InstallAsActiveX.bat