How to add images to existing PDF file in Photoshop?

I have a PDF of a purchase contract and I need to add an image of my debit card to it. So far, I've opened the image of my card in a separate file and used the magnetic lasso tool to select it. In another window, I have my PDF opened. I've tried using the "Paste Into" command to paste the image into an rectangular area I've selected, but two things happen--

1. The image of my card is automatically greyscaled.


2. I cannot transform the image at all. I would like to shrink it so it fits the dimensions of the layer mask.

I've noticed that the paste into command is not creating any new layers. Could this be because I'm opening a PDF? Should I be converting the PDF to a different format? If so, how?

Thanks all.

Photoshop CC 2014 (all updates installed)