I second the flat shoes, also helped my knee pain. I also like my cervical neck device, which helps move tension out of the neck and back and can help with lower back pain. Don’t forget to check your chair or your mattress, too. My philosophy is to always invest money in the things that separate me from the ground. You can also work on strengthening your TVA to help support the lower back, as well.
Yet another plug for Calibre. Best I’ve found for customizing and troubleshooting PDF conversions.