you are downloading it as a pdf file most likely
those are not meant to be edited but only viewed
you can purchase a program that edits them such as adobe acrobat
or what might be easier is to download it in csv or excel format which can be loaded and edited by excel.
why do you need to edit a statement ?
it wont change anything at the actual bank.
what kind of error do you get trying to edit it?
you might be better off taking a screen shot of it and editing the picture and putting it back into pdf format.
or downloading the data in excel and creating a pdf out of it.