How to download data from AS 400

Over a standard TCP/IP network?  FTP.

http://www.ortizonline.com/publications/FAQ_FTP_AS400.htm

You can use SSIS to fetch data from AS400 to MS SQL Server.

AS 400 offers ODBC and OLE DB drivers so you can access it from almost anywhere by any programming language which supports ODBC or OLE DB access, e.g. C#, FoxPro, VB etc.

Many drivers exist, you may look e.g. here: http://www.hitsw.com/products_services/odbc/odbc_for_i/odbc400.html

Microfocus also delivers drivers  https://www.microfocus.com

Of course, you should preferably ask your AS400 support for more info.

Connections strings are described here: https://www.connectionstrings.com/as-400/

We use a linked server.  Create a linked server(s) pointing to the AS400 you want to access.  Naturally you'll need an id on the AS400 that has access to what you want to read / write.

Then you use it in SQL just as you would any other linked server.

One time, or a repeated automated process?
IBM AS/400 DB2 data (tables/physical files), or flat files from the AS/400 IFS?
What format do you need the data in?

How to download data from AS 400?

As you can see for yourself, there is a large number of tools, protocols and techniques available. Some were not mentioned (like ssh/sftp). The key parameters are:
* Is it a one time download?
* What size is the download?
* What is the data?
* Does it need conversion?
* How secure does it have to be?
* What platform is doing the downloading?

This suggestion will help