What are the commands used in autocad to convert dwg to pdf? Please help..

Either [CMD]PLOT[/CMD] (to the printer "DWG To PDF.pc3") or [CMD]EXPORTPDF[/CMD].

Please elaborate these commands..how to use these for converting through windows command prompt?

You have to be in AutoCAD to use either command.

When I convert Autocad to a PDF file, the plot crashes. Any suggestions?

Find, download and try CutePDF instead.

u can use primo pdf if Plot 
to the printer "DWG To PDF.pc3" does not work.....

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You can use ClassicPDF editor for the conversion, it can convert nearly 500+ printable files into PDF, it just installs a simple printer driver during installation, and then you just have to click print for any file and select classicPDF as printer and click print and then file is converted into a PDF file. I hope this works well for you!

you can print only, on the printer/plotter name select DWG to PDF, easy.
regards