To convert a single file you can use
/System/Library/Printers/Libraries/convert -f path/to/source.file -o destination.pdf
If you want to convert in bulk you can use AppleScript.
Open Script Editor from /Applications/AppleScript, then in a new script copy and paste
on open input_documents
repeat with this_document in input_documents
set this_document_path to POSIX path of this_document
--display dialog this_document_path
do shell script "/System/Library/Printers/Libraries/convert -f " & quoted form of this_document_path & " -o " & quoted form of this_document_path & ".pdf"
then from File Menu select Save As. In File Format select Application. Give it a name, and then you can drag and drop your files onto it, and hey presto converted to PDF.