Plain text cannot be formatted and therefore requires a lot less file space.
This can happen, when the object had been moved by more than two systems from its anchor note.
Right-click the object.
Besides those objects which are anchored to the page and thus have special properties, each object has to be anchored to a note/rest or a group of notes/rests.
Please check first if the section to be exported should be selected differently.
capella does not have a direct PDF export but will be equipped with one in a later version.