Reference management is the system of organizing your reference materials and information in a way that can generate o
rganized lists, such as bibliographic reference pages. Some reference management systems include the ability to download, save, and organize PDFs and other reference sources. There are various reference management systems available and you can choose which system works best for you.
There are other reference systems but these three for which we have provided guides have free levels with free online storage (up to a point) and the ability to add more storage if necessary.
Visit this link for a comparative list of Reference Management Systems