Editing existing pdf on Samsung Tablet

When attempting enter text in editing pdf mode, the lower existing text moves down and overlaps onto other text. This happens when I tap on the pencil icon then tap on the area I want to enter the text in and a box appears on the area. When I instead tap on the pencil icon then tap on the enter text icon at the top of the screen, I cannot place the text at the margin and gives me a very small area to enter the text