best way vs easiest way---assuming the smaller size is proportional to the original:
the easiest way is to export a PDF--then 'place' the pages at reduced size in a new document (the type would be the 'same size' [reduced] but the same in relation to each other)--however--should you wish to 'edit' the file--you would need to edit the original then re-export another PDF and update the links
best way-is to rebuild the document at the proper size--keeping the text size same as original
The current document is height x width.
The type is 12pt (perhaps).
Do you want the new document to be 12pt or scaled (proportionate) to the new doc size?
The text can be scaled for the new doc size.
Than as tman69 suggested, create a new document to new size.
PDF the original doc. Place it scaled into the new document.
The MultiPageImporter script would be helpful to place all pages at one, scaled.
We have an InDesign add-on available that will rescale your document to whatever proportion, or specific size, you require. (It works like the "resize image" dialog in Photoshop, if you're familiar with that, but modifies the entire InDesign document perfectly, keeping the document fully editable like the original, of course.)
Especially if this is something you need to do regularly, it's probably the way to go. If you're interested to try it out, drop me a note here: http://www.id-extras.com/contact