I would hesitate to let any script run rampage on this.
By far the easiest way is one of printing the document to PDF and placing that, or directly placing the original ID document as image, into a new document of the correct size. But that restraints you to proportional sizing only -- your 50% should work fine, but if you enter a different scale for horizontal and vertical, you will see text and images distort. And (since you might ask), no, a script would not help here.
There are a couple of scripts that place entire PDFs automatically (search in the script repository).
There is no easy way to reformat a document. The only help you can get is by enabling Layout Adjustment, and its use is very limited. It does its work flawless when done with plain text only (say, a novel), but as soon as you have images and/or tables in floating boxes, it does resize them (in a haphazard way -- tables are not re-fitted) but no way it can keep them "on the same page as the reference", or something like that.
If you are looking for the quickest way, and don't care about quality, go for proportionally resized PDFs (using a script). If you do care for some quality, do not resize them out of proportion -- and you can still use a script. But if you want to do any editing at all -- no matter how tiny -- you will have to resize and reformat the document manually, no way around it.
Thought that maybe the case - thanks for confirming.