Paste a PDF onto another PDF or dwg. Not a combine but paste onto.

I have an Architect seal, with signature, on a hard copy.  I want only this 3" x 3" image to be pasted onto a drawing that is created as a .dwg but can be plotted as a PDF.  I can scan the 3" x 3" image as a PDF and I can save as a word document, if desired.  But the image is in the corner of the 8 1/2 x 11 white sheet and it needs to be cropped to the 3" x 3" size. Finally, the 3" x 3" image has to be pasted onto the drawing file that can be either .dwg. or PDF.  I am a new user and know little about Adobe, but I have been fumbling around with no success.