Dragging sometimes doesn't work to select text. Under Edit, use "select All" (ctrl-A) to copy one page at a time, then use copy and paste it as normal.
If Select All doesn't work, try File, "Export document to text".
If they dont work, it's locked and your need a more specialised tool. I presume this is casual use and you're not interested in paying for that, but if you are you can use something like
pdf2txt.