Q:

opening a pdf in photoshop and then saving removes margins

I wish to open a pdf in photoshop , remove some content and then resave it.  When i do this the new pdf has trimmed margins. What is the best approach to keep the margins the save as the original file. I appreciate indesign or other software would be a better for this job 

A:

When you open a PDF with Photoshop you have a choice over what the bounding area is.

Simply choose the correct bounding area to retain your borders. Most often it will be the "Bleed Box" or "Media Box" if you want the full size.
Bounding Box and Art Box are generally the size of only the artwork in the PDF.
Trim Box and Crop Box will crop to the marks. 
Bleed Box will crop to the bleed marks if there are any. 
And Media Box will crop to the full size of the document, ignoring any internal marks.

