Best tool to load data

Hi community,
I have used XML, txt to load data till now but right now I am making a mobile app for my client and I need to load huge sorted data from web and he has to understand it, change it.
So the best use for him is Excel and then txt and then xml.
Data will be huge so the speed and the disk space may matter, also I prefer the simplest way for him to use it. My question is what are the advantages and disadvantages to load data from those platform in data loading speed and memory/disk space wise?
I guess many people would like to know it from community. Thanks for your help from now!

Here is what I would do (and actually have done in the past):

Your client likes and understands Excel - great, stick with Excel
Write a converter that converts the source Excel sheets into whatever format you prefer for your app (see below). You probably need a converter in the other direction as well, i.e. create or update an excel sheet from your own data format.

Excel has a decent .NET API that allows you to read and write Excel documents easily. Microsoft has a nice introduction: How to automate Microsoft Excel from Microsoft Visual C#.NET - Office | Microsoft Learn

Internally your app can use XML or a binary format. XML seems to be the natural choice to send data over the web (but you probably want to enable compression).
Storing and accessing huge amounts of data on the device is a different matter since memory and disc speed is limited. A real database may be right choice here. Check out SQLite ( SQLite is C, so it can be compiled for any platform and there are decent C# wrappers available.