Hi phuc_trantan,
The files that I send out are zipped, which is a compression of the file. To “zip” a file means to take the original file and use an application that can compress it. Compression is (simplified) a try to find patterns or repetitions in a file structure. If an image has hundred red pixels, all the same, a compression would say 100x red instead of red, red, red, and so on. This helps to get files smaller. If there is a pattern, then this would be handled similarly.
On the other hand, the zip format is often accepted. Standard file formats are not. A nice side effect is, if the zip file opens, it arrived as it should, not compromised, and the de-compressing reveals an untouched original file.
There are other compression options available, but I do not touch them at all. Some are open source, and they might contain code that can harm one’s computer.
All the best