As far as I know there is no way to hand a memory stream to any existing PDF viewer app, so you would have to use a PDF viewer component within your application. And I wouldn;t expect such to exist, given that a stream is part of the memory allocated to your app, and that isn't normally available to external applications.
There are some available which will do it:
Free PDF Viewer Component - Read/View/Print PDF in C#,VB.NET | Spire.PDFViewer[
^] provides a "community edition" which is limited to 10 pages but which can work directly from a stream. There may be others, but you'll have to work with Google to find out.