Damn, that's a sneaky way to scam someone and the one that is very easy to fall for if you are doing business on Telegram as there is a lot of stuff going on there all the time. I know its not the mistake that you specifically did, but I would advise everyone to disable the option to automatically download any files as this thing is enabled by default on Telegram and I heard many stories about people getting rekt that way.
Telegram is risky as well as useful at the same time so we need to be extra careful while dealing with the third person whom we don't know personally as this is pure risk dealing and you need to have all the safety measures.There are number of telegram scam cases like adding you to some fake groups and now this auto download.So for those who don't know how to turn it on here is the procedure for you all
For not being added in any group without your permission:Telegram>Privacy and security> Groups>Who can add you to group> My contacts
You can add exception to these cases anytime
For turning off the auto download meadia features as sometimes it is by default on in Telegram:
Telegram>Settings>Data and storage> Auto download media turn off on mobile as well wifi network
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This is sad to hear about that despite being aware about these crypto scams you unintentionally fall victim of it losing your hard earned money worth $12k and know how hard this must be for you at this time.But these scammers are always trying to find an opportunity which they gained in your case with this phishing PDF file and having access to your metamask wallet draining out the funds out of it.
Recently as per reports the number of phising PDF scams have grown drastically over the past two years as scammers are finding out the new ways to make the fool of people.Mostly they are carried out by redirecting you to some fake websites and giving the hackers full control at the backend like installing some keylogger malwares and other virus that can access your files over the system.As per study these scams are classified into various types according to their percentage as follows:
You see they have various ways to make you fool like by verify the captcha and other ways.As you said it was Screen saver scam so the hackers just sent you clickable pdf which was gaining access to your system and funds were withdrawn.
Generally as said they are injected with with malwares that are stringed to hackers backend giving him the access to your wallets as Metamask is usually logged in your system as browser extension so it's easy once they gain access to your system.Here is one reference image explaining how it will redirect you to some fake websites:
You must be thinking what's happening while the bad actor's are draining out your wallets and once you know whats happening and revoke the permission it's too late as happened with you.This is happening nowadays for the NFT's people having on Metamask wallet but you also haved funds over there.This is why you need hardware wallets and keep as low funds you could on these wallets for you safety.
Read this
article to check more about such scams in detail and be safe from them in future.
Take it as lesson and be safe i future.