Muriatic acid will not harm PVC pipe, which is commonly used for drain lines. However, this does not mean it's a good idea to pour this acid down the drain in order to dissolve a clog. \
Professional plumbers do not use chemicals to get rid of clogs in drains. They use tools, in particular a plumber's snake, in order to push the drains out of the pipe. \
You should never pour chemicals, not even chemical drain cleaners, into your pipes even if the chemicals will not do any damage. Chemicals like this can get into the groundwater and into sewage systems, which is bad for animals and plant health, not to mention human health. \