netDxf[
^] can read DXF files and it supports Polylines. I don't think it supports conversion from DXF to an image, but if you can at least read the file, you can probably extract and draw what you need.
I'm also not sure if it supports .NET 3.5 -- the readme file mentions .NET 4.5. You'll have to check if it also compiles for 3.5.