I often have to send a lot of files - without archives you can’t go far when you have to transfer many files at once. I have been using the zip-archiver built into Windows for several years to create archives, since zip opens today everywhere without installing additional software. However, there are such incidents that my customers send archives to rar and demand to answer archives in the same formats. Here, it would seem, any rar program like winrar can come to the rescue, but recently it did not cope with some of the files that I received the other day. I also tried to open via paid - 8zip, which is able to open many archive formats, but nothing happened. What do you recommend to use from the software? I would like to have a program for such cases of life, so as not to force customers to wait and not be nervous themselves.

7zip to the rescue! Open source and free

Try this:
Open the file in WinRAR (download from internet if you don't have one).
Press your right mouse-button.
Click on "Repair archive", or press Alt+R.
Click on Browse button and select a folder where you want to keep the repaired archive.
There will be two options ("Treat the corrupt archive as RAR" and "Treat the corrupt archive as ZIP".
Select any one of your choice (or see which one will work) and click on OK button.
Go to the folder where you have kept the rar repair and open it (Repaired archive will be named as rebuilt.*.rar or rebuilt.*.zip, where * is the name of original archive).

https://in.answers.yahoo.com/question/index?qid=20120413024756AAkXcMu
http://www.winrar.repair/