Webbrowser control in Winforms, showing .pdf - sort of

I programmatically create a Webbrowser control in a form to show the contents of a pdf file - it works, but one thing is strange - the pdf appears in the upper-left-hand corner of the Webbrowser, using half the height and half the width of the control (it shows up with scrollbars, so it's still readable).

In the attached image, the white area is the webbrowser control (the form background is dark gray, I didn't include much of that).

The fact that it's using exactly half of the height and half of the width seems to be a clue, but I can see no property or setting that would affect this. Does anyone have any ideas?

Attached Images