Dunno if it's normal, but I get it too.

Post screenshot so we can know what you are seeing.

Can you modify the icon through ObjectDock to something better?

I don't use Object Dock, but I do make docks. So I thought I'd try to help. 

Well, there was that work-around but you will need to do that for each file.  It seems to me that OD should use the icon included with the program. It's right there in Bundled Images.

Ahaha. Zubaz, I blame Adobe too, as I've run into this myself.

PDF thumbnails are unique in that they have all the alpha bytes set to 0, so the image renders fully transparent or all black. Before converting the GDI bitmap of a thumbnail into a GDI+ bitmap, I make a quick diagonal scan of the thumbnail: if all alpha bytes are set to 0 I convert using GdipCreateBitmapFromHBITMAP, which throws away alpha information:

            ' Do a quick diagonal scan to make sure not alpha bytes are set to 0.
            ' Otherwise PDFs thumbnails and videos thumbnails under XP are rendered
            ' as fully transparent. If all alpha bytes in our scan are set to 0,
            ' then render using GdipCreateBitmapFromHBITMAP.
            
            Do While X < BMInf.bmWidth And Y < BMInf.bmHeight
                If BMData(Y * BMInf.bmWidth + X * 4 + 3) Then
                    IsAlphaBitmap = True
                    Exit Do
                End If
                Y = Y + 1
                X = X + 1
            Loop

You might want to tell Jeff about this, assuming he doesn't know about it already.

I'll second that.