When a CAD drawing is saved as a PDF file, it still retains information about layers that can be extracted. When converting, pdf2cad looks at color attributes, or other definitions in the PDF file, to create layers. If the option to recognize layers is turned on, pdf2cad gathers all objects with same color and put them on one layer. If the PDF file has three colors, the resulting DXF file will have three layers. It is then easy in AutoCAD to turn off a complete layer. This setting can be found in the Options->DXF tab of PDF FLY or simply under Options in pdf2cad.