To read college level mathematics, do not plan to skim it quickly like you might read a novel for pleasure. You will need to read slowly and reflect upon what you have read. Look at the examples and drawings. Try the suggested practice problems after each section of reading to ensure that you understand that topic. Read the material from the computer screen and the text to help ensure your understanding.
Many people find that it is easier to read online material after if has been printed out.
The print seems clearer to many of us when we read it from paper rather than a monitor. Consider printing most chapters to read. I found that I could read many printed pages of online courses while waiting for my kids at ice skating classes and baseball practices. I also wrote in the margins of the printed online lectures my own understanding of what I had just read. Writing a description of what I had just read in the margin forced me to think about what I had read and put it into my own words.