Get PDFill from here:
http://www.pdfill.com/pdf_tools_free.html
1 - launch it and go to "Encrypt and Decrypt Options"
2 - open the file and enter the password
3 - uncheck all security options and "Decrypt As ..."
4 - save the file and close the encrypt & decrypt window
5 - go to "Convert PDF to Images
6 - open the decrypted file, insert an Image Resolution of 300 dpi and "Save As Image ..."
7 - open the images in Gimp and resize them by 300 % : Image > scale, save the image and close the image window
8 - open the rescaled image in Gimp and use the crop tool to chop it into suitable pieces which can be printed on A4, A3 or letter size paper.
I know this is a very labour and time intense task but it is worth it if you would like to get a model in the right scale. I do not recommend a poster-printer because it is not guaranteed that the parts are really printed at 300 % in the end. In many cases the image comes out a bit smaller because the printing program adds a rim to it for glueing the single parts together. So you may end up with a 5 - 10 % smaller image (depending on the program).
You may ask your local copy shop if they are able to deliver a 300 % output. But be careful; in many cases the prints will cost a small fortune and sometimes the prints come out in the wrong scale (I know of at least one case here where both of this happened).
Please note that the attached images are not displayed in the correct order. They are numbered sequentially and have to be seen in the correct order from 1 to 8 .