You might look to see if there is anything checked in the options when saving as a pdf file that could be causing that behavior.
Since the plane extends beyond the view's "box", I suspect it isn't a view outline/boundary, but perhaps a sketch leftover from a deleted crop view?
Can you select the pieces?
If you can, please upload.
Check your layers.
Also, does it print to paper?
PDF vs paper aren't always the same. What's invisible on the layer's properties doesn't always translate to a PDF.
It was do to a cropped view, funny it only did this on one cropped view of several in the drawing.
Even stranger is that the box that printed did not have the same dimensions as the box used for making the crop.
I just ended up modifying the display state of the model and getting rid of the cropped view altogether.
Just another of the many bugs living in SW