Doing a google search to find code which can display PDFs in a vba userform revealed some hits but they appear to not be totally reliable so the performance might be hit or miss on your machines.
If you are using a Application.FileDialog(msoFileDialogFolderPicker) to build a list of PDFs, why don't you use the FilePicker instead and let Windows Preview take care of showing the preview of the PDF? See
FileDialogFilters Collection | Microsoft Docs. This way you just return the chosen PDF to the userform (or avoid the userform completely if that was all it was for)