Hi dippy and welcome,
the only time I get the symbol you describe is when I am trying to go to a page that doesn't exist - e.g. didn't realize I was at the end, or swipe in a way reader doesn't recognize. Only time I had trouble w/font size was on a pdf and the file was corrupt.
Calibre is awesome for managing your books and conversion from pdf to epub is usually quick and easy. Make sure that if you keep the Sony software you disable the auto sync with your reader. You can't have your reader syncing with the sony software and calibre at the same time (they don't play nicely together

)
Happy reading!