Amit,
If you are looking only for opening pdf files, then an assembled machine with I5 Processor, with a min 6GB Ram, Nvidia Quadro min 512MB graphics card may help. I am assuming that you need to open A0 GA drawings, and unit/block drawings, system drawings etc.. in PDF format just for checking. I am using similar configuration for same applications and buy ViewSonic large display monitors. Should cost you around 65K without OS..
I am sure you will need AutoCAD installation in your machine since reviewing including editing work requires you to open large ACAD files with 20 / 30 MB size with smooth pan and zoom, then look at entry level HP Z200 (cheaper) series or Z400 series mid range certified for ACAD. There wont be much cost difference and some times will almost be equal with good warranty agreements. You can nego for some good discounts as Workstation business is handled by separate channel and they offer some fabulous discounts now due to low numbers...
You can even ask for a demo of their mobile workstation line which also handles CAD drawings and 3D CAD applications. but max screen size will be limited to 19"..
You need to check with HP and Dell. Dell offers better discounts these days.