Knowing how to decrease file size will surely make it easier for you to upload them. You should first find out about your file type and get rid of everything that is not needed. Big images usually can be compressed or resized, while PDF files tend to shrink once you remove unused pages, pictures, etc. In case you have to work with a video, try lowering its resolution or bitrate. There is also a tool for file compression which will help you if you need something fast.
