If you receive an error while uploading documents, this is most likely because the file you are trying to upload is too large.
Uploaded files should always be smaller than 2MB. JPEG or DOC file types will typically fall within this range. PDF files sometimes can be too large, especially when they are produced from a scanner. If this is the case, please either export the PDF as a JPEG or reduce the file size.
As an example, here is how you can reduce the file size of a PDF on a Mac with Preview: