There are three linked resources under Differential Equations. All the PDFs (or the most recent if there were multiple) from each downloaded just fine for me.

There are three linked resources under Differential Equations. All the PDFs (or the most recent if there were multiple) from each downloaded just fine for me. For Trigonometry check the page again. The page it links to (for the one simply titled "Trigonometry" author Katherine Yoshiwara) there are multiple books available. The second book on that page is Trigonometry. Modeling, Functions, and Graphs is the first book, just below it is Trigonometry: https://github.com/byoshiwara/trig/blob/master/trig.pdf https://github.com/byoshiwara/trig/blob/master/trig.pdf.