I suppose this is empty content. It is probably caused because the source document was not prepared for conversion to an accessible pdf, or because the source program (like a word processor, PowerPoint or desk top publishing program) did not do the conversion to a pdf perfectly. An example are empty paragraphs caused by blank lines in the word processor file.
While I do not dare to call myself an an expert (yet), in my experience you can wander through the objects ('bank boxes') and see if somewhere on the page a purple box marks the content. You can also clap use the mous or the right cursor key to see if there are any sub elements. If there aren't, or it is only a space, you can safely delete them. Marking them as an artifact is also an option.