pdfView pinch zooming out

Just to confirm that the accepted answer is the correct one, but also to highlight where the code needs to be used (as stated in the comment by answer author). i.e. the code must be used AFTER setting the pdf document:

pdfView.document = pdfDocument
pdfView.autoScales = true
pdfView.maxScaleFactor = 4.0
pdfView.minScaleFactor = pdfView.scaleFactorForSizeToFit 

to answer my own question, it was actually very easy...

pdfView.autoScales = true 
pdfView.maxScaleFactor = 4.0 
pdfView.minScaleFactor = pdfView.scaleFactorForSizeToFit