Just set up Linux Mint! How can I expand my software options?

I've finally got Linux Mint installed, and I have a beginner's question about installing software. What's the best way to increase the number of applications available in the Software Manager? I wanted to install Foxit PDF Reader to fill out PDF forms, but I couldn't find it when I searched. Here's a bit about my setup: I have an i5-7400 CPU, a GTX1050Ti GPU, 32 GB of DDR4 RAM, and a 2 TB SATA HDD. By the way, I did find an alternative to Foxit, but I still want to know how to get more apps in my Software Manager!