Taking an antibiotic as a prophylactic (ie: to prevent infection) before an abortion with pills is not recommended. The risk of infection with medical abortion is very low (less than 1%).
If you develop an infection after an abortion (for example, you get a fever or a smelly vaginal discharge), you should go to hospital. Do not try to treat yourself with antibiotics you have at home. This is because antibiotics on their own are unlikely to be enough - you will also need to have an ultrasound and any remaining pieces of pregnancy will need to be removed.