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 But storing it offline would mean that it becomes out of date very quickly and thus missing new transactions.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.
When you 'own a bitcoin', what you actually own is a private key that can be used to spend the bitcoins that are located at the key's corresponding bitcoin address. Bitcoins themselves do not technically exist, instead the network simply keeps track of 'unspent outputs' at certain bitcoin addresses. Whoever owns a key that creates a valid signature to spend an unspent output can be said to "own the bitcoins at that address".
With that in mind, a wallet file is a collection of such private keys, which can be read by the wallet software so that you can utilize all of the wallet's functions.
However, it is possible to store just the private key itself in any format you desire. A private key is really just a very large random number, it can be encoded and stored in any number of ways. Keep in mind, if anyone is able to get a hold of the private key, they can steal your bitcoins. So it is a good idea to store the private keys in a very secure manner!
If you want to use the private key to spend your bitcoins (or perform any other function associated with a private key), then you would need to import that private key into some sort of wallet software.
is it possible that BTC itself could be stored in a PC independently without a wallet
Reading carefully, I think that you may mean keeping the Bitcoin without a wallet software.
If you install wallet software it will create a wallet file. In the case of Bitcoin Core this file is named
wallet.dat. I can set a passphrase for my wallet, transfer some BTC into my wallet, exit Bitcoin Core, copy this file to my documents folder or somewhere (and rename it if I like) and then uninstall Bitcoin Core completely. This is risky for several reasons.
If you rename the file to be .doc and accidentally open it with a word processor program it may become corrupted. Also, you may forget what you named it. You should always set a passphrase to prevent theft and keep a secure backup copy of your wallet file.
Without a wallet software, it is not possible to use or spend your Bitcoin.
You may enjoy reading about a paper wallet with several options which may be suitable for your needs. You can receive BTC with your paper wallet but in order to spend you once again need a wallet software. The information is on the website. Caveat: I have not used this service.