I am desperately looking for a Pdf viewer which can show the pdf without any margins, that would help me a lot in viewing two pdfs simultaneously.
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Cropping the page could work for you.
So, open up whatever PDF you want to crop in Preview. Hit the Select button in the toolbar and crop the first page as you want all the selected pages to be cropped. The cropping tool works just like any crop tool in your average photo editing software. Then, in the sidebar on the right, select a thumbnail of any page (make sure the sidebar is in thumbnail view and not Table of Contents or Annotations view). Now check to be sure that the crop size is what you want to be applied to all the pages you select, then hit Select All, and go to Tools » Crop or hit Command-K.
Here is the link how to crop all pages within a pdf.
Uncrop see here