Text boxes changing after saving and re-opening PDF

I have been formatting a PDF document, using a couple of text boxes on a page. In some cases an individual text box will have a couple of types of formatting e.g. a bold heading and a bullet point list below. After saving and closing the document, I wanted to make more edits to the text. I re-opened, went to the 'edit' option, and all of my text boxes have disappeared - there were 3 in a row, there are now about 10, all with different portions of the original text in them. The allocation of text in different boxes is quite random. How do I stop this happening? It makes using this edit function pretty redundant if I can't fix this!