Under international copyright law, no computer drawing except the Babbage Engine will have yet entered the Public Domain. The copyright lasts for the time specified by the law in the country of origin. So for anything coming from the USA, it will most likely never enter the PD. That is due to the ridiculous extension of the term of copyright to protect Mickey Mouse and other Disney things. (That is the same Disney who produced the infamous and completely bogus Lemmings film, complete scumbags.)
If it originates in the UK for example, the term of copyright will be to the end of the 70th year after the author's death for a literary work, but somewhat more complicated but no longer in duration for corporate works.
In many countries there is no concept of Public Domain, and elsewhere it has no legal definition, so I suggest being careful about its use.
The UK has fairly recently set about fixing the problems with copyright on "orphaned works". There is now a procedure involving making honest attempts to trace the copyright oner, before you get to use it freely. The EU meddled and prevented us getting what was actually needed.
Just because a company is defunct does not mean that the copyrights are not still owned by someone. The liquidator, bankrupcy trustee, or whatever he or she is called in various countries, should have tried to sell off all the assets, and that includes copyrights. If the owner dies, it goes to his estate, and probably children and grandchildren, for 70 years. If he has no inheritors, all assets go to the government. There are copyright trolls, an even worse form of scumbag than patent trolls, who seek out and acquire copyrights on old, and not so old, things with a view to basically holding people to ransom. So it is entirely possible that there are huge problems looming for the large number of people who, often of necessity (and that includes me) have obtained illegal copies of documentation and such like, just to be able to use or repair vintage equipment, including computers, cars and many other things.
In some cases the risk of copyright enforcement is minimal, but not guaranteed. What is essential, as has already been said, is to avoid any commercial activities such as selling copies, because that can move the copyright offence from civil (where they may sue you in court) to criminal, where you can be arrested by the police, tried, and fined or even imprisoned. Personally I think that the commercial copyright violators, typically DVD pirates, deserve what they get, but I don't know of any country except the USA (where fascist organisations like the RIAA are getting penalties of maybe a million times the true worth of the violation awarded in court) where non-commercial copyright violation would attract more than damages of 4 or 5 times the worth of the copied material, usually not worth a court case. But if you do get a "cease and desist" letter from a copyright holder's lawyer, best to respond to it immediately.
By the way I use the term "fascist" advisedly, as one of the 5 main pillars of Fascism is apparently privatised law enforcement, and that is exactly what these guys try to do. Don't get on the wrong side of them.
But, as already suggested, using private (not visible to the public) copies of things for decorative purposes is unlikely to attract the attention of any copyright holder. Just don't post it on a blog...
You should never take any action that can potentially get you into trouble based on advice given in an internet forum. Only your own lawyer can give you definitive legal advice for your particular circumstances. That is a universal rule.