Can Bitcoin be stored in a computer without a wallet , just like a normal file? Everything on computers are stored in some type of files like .txt , .MP3 , ... Etc , so is it possible that BTC itself could be stored in a PC independently without a wallet , just like any other file on a PC or a phone ? Or is it compulsory to store it in a wallet , so that one couldn't make offline transactions , as that wouldn't be added to the ledger ?
On the technical level, there is no such a thing as a Bitcoin. What there actually are are transaction outputs. Transaction outputs consist of a output script and a value. The value is in satoshis, and that is what gives your wallet a "balance". The output script specifies a set of conditions  that are required to spend the value associated with that output.

Typically the output script requires that a digital signature be made with a public key whose hash matches a hash provided in the output script.

So, no, a "Bitcoin itself" does not have any associated files that you can store. However you can store the Bitcoin blockchain (thus store the transaction data, including transaction outputs) locally, and that is what nodes do. But storing it offline would mean that it becomes out of date very quickly and thus missing new transactions.

Alternatively you can store the private keys which are necessary to spend from the typical type of transaction output. These can be stored independently of any wallet software. But you still need some software to create a transaction and then broadcast it to the rest of the network. In order for a transaction to "go through", you must broadcast the transaction so that it can be added to the blockchain. Just making it offline won't do anything.