Resizing a given paper model is always a bit tricky... :-|
First we need to distinguish between two types of PDFs:
- PDFs that contain pixel-based images: Each page is an image consisting of a great number of single pixels, each with a specific color, aligned next to each other.
Resizing pixel-based images always results in lower quality because the images got a fixed number of pixels and are only sharp in 100% size. Scaling them up results definitely/scaling down mostly in blurry images.
- PDFs that are made of vector-based images consisting of lots of single items such as dots, lines, circles, rectangles and/or polygons from which - in combination - you can build all kinds of forms. The main advantage of vector-based images is that they look always the same - no matter what size you scale them up.
Resizing vector-based images is easy and produces the most usable result.
To determine what kind of PDF you´ve got just zoom in to 500% for example. If the single parts of the paper model are still sharp it´s mostly vector-based. If you see big single blocks forming the single parts it´s a pixel-based image.
First of all you can try opening the PDF file in a graphics application like Illustrator (commercial) or Inkscape (freeware) which can handle vector images and with which you can open single pages of the PDF (or at least extract them). After that try to select the single parts. If it is a pixel-based image then the whole page will be one item. In a vector-based image the parts may consist of different pieces (lines, fillings, ...) which also need to be selected. After that scale the selected parts to whatever size you want.
Regarding final size of the parts:
- Scaling down and combining parts of different pages in one single page makes it necessary to open all the necessary pages and copy all the parts in one page, arranging them afterwards so they don´t overlap each other.
- Scaling up may call for splitting parts (given on a single page in the PDF) into two or more separate pages. Because of their new size they might just don´t fit anymore).
After that you can print the result and start building your model.
If you didn´t already fall asleep
I hope this sounds at least a bit reasonable to you.
(Think I might write a little guide about it...)