JPedal provides several methods to allow easy conversion of a PDF file or directory of PDF files into BMP.
Convert PDF to BMP from Command Line or another language
java -jar jpedal.jar --convert "inputFileOrDir" "outputDir" bmp
Convert PDF to BMP in Java
writeAllPagesAsImagesToDir("inputFileOrDirectory", "outputDir" ,
This uses the JPedal ConvertPagesToImages class.
If you are looking for upscaling or more complex control over the PDF to BMP conversion, ConvertPagesToHiResImages class has lots of additional options