ShellBrowser Delphi Components Documentation
If set to true PDF files will be previewed via the Acrobat ActiveX object if the registered PreviewHandler failed.
property FallbackPdfHandlerActive: Boolean;
The fallback is only used when the registered PDF PreviewHandler fails. Also it will only work if the system has Adobe Acrobat installed and the application is 32-bit. Due to these limitations the property is false be default. You can activate it if you experience problems with previewing PDF files.