SD card writing and reading a large amount of data using SPI

I'm writing JSON strings to an SD card when there's a internet failure . When i'm trying to fetch the data stored on SD card and post on to a cloud storage, when number of characters are more in number the variable defined as 'String' is unable to store those amount of characters. So, say there are some 20,000 characters are stored on SD card and i want to read all of them and post it to cloud - how can i be able to do it?