You will need a PDF renderer. There are a few more or less good ones on the market (ICEPdf, pdfrenderer), but without, you will have to rely on external tools. The free PDF renderers also cannot render embedded fonts, and so will only be good for creating thumbnails (what you eventually want).
My favorite external tool is Ghostscript, which can convert PDFs to images with a single command line invocation.
This converts Postscript (and PDF?) files to bmp for us, just as a guide to modify for your needs (Know you need the env vars for gs to work!):
pushd
setlocal
Set BIN_DIR=C:Program FilesIKOffice_ACMEin
Set GS=C:Program FilesIKOffice_ACMEgs
Set GS_DLL=%GS%gs8.54ingsdll32.dll
Set GS_LIB=%GS%gs8.54lib;%GS%gs8.54Resource;%GS%fonts
Set Path=%Path%;%GS%gs8.54in
Set Path=%Path%;%GS%gs8.54lib
call "%GS%gs8.54ingswin32c.exe" -q -dSAFER -dNOPAUSE -dBATCH -sDEVICE#bmpmono -r600x600 -sOutputFile#%2 -f %1
endlocal
popd
UPDATE: pdfbox is now able to embed fonts, so no need for Ghostscript anymore.