unable to make the background transparent

The white background in the PDF is the "page" that was baked into the document (by the iLovePDF app). When I open the PDF in Illustrator the page is made up of a grid of white squares. If you move the contents of the PDF onto a second layer and then place a colored object on the layer underneath you can delete all those white squares and arrive at an end result that is mostly transparent. There are still plenty of objects that have fills. Another warning: a bunch of those squares are functioning as clipping masks. They contain artwork. Delete the wrong squares and you'll delete artwork. So a bunch of the clipping masks have to be released. This process can get pretty complicated and time consuming. I gave the PDF a pass thru the Vector First Aid plugin (to automatically get rid of unnecessary clipping masks, clipping groups, etc) and the plugin almost crashed. Not everything in the PDF is vector-based either. Some of the white squares contain pixel-based content.

Here's another idea: open the PDF in Photoshop. Choose the size/resolution you want for Photoshop to render the document into pixel form. The document is basically a grayscale image. So you can turn a copy of the image into an alpha channel. Create a new layer. Turn the alpha channel into a selection and then fill the selection with the color of your choice. Turn off the visibility of Layer 1. Save for web. You'll have a transparent PNG.