Peter,

.pdfs and other file types are accepted, see screen shots below.

Click image for larger version. 

Name:	IVA screenshot file types.jpg 
Views:	72 
Size:	39.5 KB 
ID:	18056

Click image for larger version. 

Name:	IVA supporting docs screenshot.jpg 
Views:	69 
Size:	40.5 KB 
ID:	18057

Also it's worth checking that you need invoice and parts evidence for IVA (vehicle safety), I thought that was part of the DVLA (vehicle registration) process.