You have two options to change the physical page size of your PDF pages. Option 1 is to run the 214 pages through the Adobe PDF print driver after selecting a custom page size according to your desired output specifications. You will also need to select the Fit or Shrink to Printable Area option in the drop down menu of the print dialogue, which will give you a preview of the content in the new output format. The downside of this route is that it will subject the document content to some degree of image degradation, which may or may not be acceptable.
The other option I'm aware of involves a third party plug-in from Evermap called AutoPagex. It is unique in that it will alter the existing PDF to conform to your desifed specification, and avoid the image degradation issue. It also permits you to proportionally scale the PDF page content, or leave it as-is. I use it routinely to convert A4 to letter sized as a workaround for some software that does not permit the output of PDFs to user-specificed page sizes.
Disclosure: I have no affiliation of any kind with Evermap, but did address the use of several of their products in my book, "The PDF Litigation Guide."
Hope that helps!
PDF Litigation Solutions, LLC