Files must be zipped before uploading. The SFTP can handle individual files, but the process takes longer than if that same data had been compressed. Uploading individual files may also cause issues should files not transfer as expected.
For example, if an index file reaches the processing server, but not all of the documents referenced in the index file reach the server, some documents are not imported. The process creates a batch with errors that has to be processed. If the same files are uploaded as a single compressed file folder, the files are unzipped on the processing server, leading to a greater chance of a successful upload with no errors or missing files.