Might sound stupid but I suffered from just this: In Ubuntu 14.04 it is not enough to install only the package libreoffice-core or -common to convert to PDF, but you need to install libreoffice-writer instead/additionally. The important thing to note is that libreoffice-core|common seems to bring in some functional libreoffice base, hence the name ;-). That doesn't error out if executed to convert some file, but instead just seems to wait on some socket or such for input or whatever... After I installed libreoffice-writer, which has [...]-core|common as a dependency, converting some ODT file to PDF worked like a charm with the exact same command line like used before.