if you want to import a pdf-file in gimp and to scale it, its not as easy and comfort then in other programs which aren't freeware. But you have full control over the process and that is a good and important point for gimp!
First of all: If you want to import a pdf-file gimp shows you a box with a given resolution. You can read in this box your pic has lala pix in x- and lulu pix in y-direction.
The next line shows that the pic is maybe imported with 100 pix per inch. That not much for your purpose, you can use a higher resolution say f. e. 300 pix per inch.
Let's say your pdf file page has a original size f.e. 8 x 11 inch: with the resolution of 300 pix per inch you get in x-direction 2400 pix and in y-direction 3300 pix.
And now there is the importing thing to know: If you want to scale your pic f.e. two times larger (smaller) , you go to -> image -> scale image and in this box you find only a possibilty to control the resolution: And here you have to take a resolution of 150 (600) pix per inch to get the right size of your pic.
If now yor pic is larger as your printer can work with, the printer driver of gimp takes a resolution that it can be printed (the other way round to make pics smaller, there is no problem). So you have to cut your big pic in smaller units that find place on a page the printer can print and that with the correct size (means resolution you have adjusted before in the scaling box) you want.
I hope that will help you, even my english is driving a native speaker crazy.