Pdfs turning green when I save them

I have Adobe Reader (the free version). If I save several pdfs in one day, the first several save and open with no issues, but at a certain point (if I save several in one day), the pdf starts to have a bright green/ blue highlight when I open them and it is very difficult to read. Is this because I have the free version and there is a max for how many I can save in one day, or is it unrelated? Thanks in advance.