The video editor used shouldn't really matter so much as the codec and the settings used to save the output video. Like JPEGs, video is usually stored using lossy compression so each time you edit and save you are discarding more data unless you go back to the original file each time. I'm not familiar with PowerDirector (I use Vegas) but if there's an option for marking the source file with where to apply editing effects (cuts, use of LUTs, adjustments to brightness/contrast, etc.) and only actually applying them when to output the file each time that is the way to go, but you can still expect to see at least some loss of quality. I'd be very surprised if PowerDirector can't do this as it's pretty much a standard feature of all but the most basic video editors.
You should also check the settings for the codec you are using to save the video (.MP4, h.264, h.265?) and make sure that everything is appropriate for your output resolution and quality expectations; there are a LOT of low quality codecs out there, and some will default to insanely high levels of compression by default and it could be a simple case of adjusting the quality slider and accepting the larger file sizes that result. Experiment with small clips until you find a codec and set of general settings that work best for you, and be aware that clips with large areas of the frame changing will generally require a higher bitrate than those that are mostly static. IMO, h.265 will offer the best output quality for a given filesize, but it can be quite CPU hungry to decode if you don't have some a recent CPU/GPU with hardware rendering support.
