To print multible files using one command

Go to the directory where the files are and open Terminal into that directory and run
for FILE in *.filetype_here ; do lpr "$FILE" ; done

An example:
for FILE in *.pdf ; do lpr "$FILE" ; done

Example 2:
for FILE in *.txt ; do lpr "$FILE" ; done

Example 3:
for FILE in *.jpg ; do lpr "$FILE" ; done