what does a red box on the page mean in Acrobat Pro DC?

i am creating a large document by combining smaller PDF files. On some of the pages there is a green box framing the entire page and inside that there is a smaller red box. Does that mean the originals are not 8.5 x 11 like the other pages? and if so, can i 'shrink to fit'?  The printed docs are for reference only as they wiil be provided the original doc on a jump drive.

thanks!

ellen