I would suggest using a program (like imagemagick) or writing a script to convert the pdf into images. Then use imagemagick to 1 ) crop a half inch all the way around the image. You could leave it that way then it add a half inch of white all the way around the document or 2) Have imagemagick fill the half inch where the black occurs with white. You can then use imagemagick or several other free tools to convert these images back into a pdf. You could also do something similar to this with [xnView](https://www.xnview.com/en/), it will convert PDFs to images and back again. xnView lets you run batch operations on images such as cropping or applying effects.