The CCEL has a very extensive pdf collection of Schaff’s Patristic texts in English (as well as his history of the Church and several other multi-volume things.) It doesn’t feature absolutely everything, and it isn’t a critical edition of anything (in the sense of providing a track of all alternate readings and rationales for choosing one or the other, although he does sometimes talk about differences between extant Greek and Latin versions or includes the Latin or Greek text for comparison). But it includes everything that existed at the time he compiled it (in the 1800s), which is still maybe 95% of what’s known today. I often use it for reference; and he’s still a respected English source for scholarly work.
I know there are couple of internet sites that feature good selections, maybe even better than his (one site has a page dedicated to including texts found since Schaff), but I’ll have to get on my other computer to locate them.