I strongly suggest you use RoboCopy as this has a wealth of options (far beyond the list I've provided). However, as you only want to copy PDF files, use this syntax
Robocopy C:\Users C:\UserBackup *.pdf
ROBOCOPY source destination [file [file]…] [options]
where source is Source Directory (drive:\path or \\server\share\path), destination is Destination Directory (drive:\path or \\server\share\path) and file is File(s) to copy where names or wildcards can be specified and default is “*.*” (all files).
Robocopy Options and Switches
Copy options :
/S :: copy Subdirectories, but not empty ones.
/E :: copy subdirectories, including Empty ones.
/LEV:n :: only copy the top n LEVels of the source directory tree.
/Z :: copy files in restartable mode.
/B :: copy files in Backup mode.
/ZB :: use restartable mode; if access denied use Backup mode.
/EFSRAW :: copy all encrypted files in EFS RAW mode.
/COPY:copyflag[s] :: what to COPY for files (default is /COPY:DAT).
(copyflags : D=Data, A=Attributes, T=Timestamps).
(S=Security=NTFS ACLs, O=Owner info, U=aUditing info).
/DCOPY:T :: COPY Directory Timestamps.
/SEC :: copy files with SECurity (equivalent to /COPY:DATS).
/COPYALL :: COPY ALL file info (equivalent to /COPY:DATSOU).
/NOCOPY :: COPY NO file info (useful with /PURGE).
/SECFIX :: FIX file SECurity on all files, even skipped files.
/TIMFIX :: FIX file TIMes on all files, even skipped files.
/PURGE :: delete dest files/dirs that no longer exist in source.
/MIR :: MIRror a directory tree (equivalent to /E plus /PURGE).
/MOV :: MOVe files (delete from source after copying).
/MOVE :: MOVE files AND dirs (delete from source after copying).
To use Robocopy is simple, just like how you would use Copy and Xcopy commands. For example, to copy entire folder of C:\Users to C:\UserBackup, simply type:
Robocopy C:\Users C:\UserBackup