Thanks everybody for lots of advice and suggestions. We don't have access to Solidworks itself, only the 3D e-drawing (.easm) that is supplied by the project engineer. We use the Solidworks eDrawings Viewer to manipulate the model to get the illustration that we need. Unfortunately, if we want to export a vector, its 'all or nothing' - which is probably why the resulting image renders so slowly on screen.
I agree that in a perfect world the engineer would create all the illustrations we need as 2D PDFs directly from Solidworks; or we would have another Solidworks licence so that we could do it ourselves. In our imperfect world, we have to make do with the eDrawing. I've concluded its best to use a bitmap wherever possible, and a vector only when absolutely necessary. Cheers, Jo