How odd. Just ran Droid 6.1.5 from the command line and it worked just fine.
$ droid q -Ns ~/.droid6/signature_files/DROID_SignatureFile_V84.xml -Nr ~/Downloads/DDM_SR_20160443_197-312_935.pdf
19:22:23,212 INFO [main] ReflectionServiceFactoryBean:399 - Creating Service {
http://pronom.nationalarchives.gov.uk}PronomServiceService from class uk.gov.nationalarchives.pronom.PronomService
DROID 6.1.5 No Profile mode: Runtime Information
Selected folder or file: /Users/andy/Downloads/DDM_SR_20160443_197-312_935.pdf
Binary signature file: /Users/andy/.droid6/signature_files/DROID_SignatureFile_V84.xml
Container signature file: None
Open archives: False
/Users/andy/Downloads/DDM_SR_20160443_197-312_935.pdf,fmt/18
i.e. fmt/18 (PDF 1.4) as declared in the header of the file. Apache Tika, Siegfried and Fido also identify the format with no problems.
Andy