Finally, the solution was to enclose the TOSTMF parameter in a double apostrophe TOSTMF(''/REPORTS/xxxxxx.PDF'')
CALL QSYS2.QCMDEXC('CPYSPLF FILE(AUMENTO1) TOFILE(*TOSTMF) JOB(557767/RAB/AUMENTO1) SPLNBR(1) TOSTMF(''/REPORTES/xxxxxx.PDF'') STMFOPT(*REPLACE) WSCST(*PDF))'