Perhaps there is an easy answer to this, but I couldn't find it. How can information be defined? It exists, but it can be multiplied indefinitely without losing anything. So that excludes it as a type of energy, since it can be created. So what is it?
(The framing seems to assume some form of physicalism, so I am going to answer in terms of modern physics. There are good philosophical reasons to question physicalism. But I do not think this is part of the question.)
The closest concept that enters physics is information = negative entropy, the ability to direct energy so that it does not disrupt an expected state.
Physics does include entropy, which measures the orderliness of a system, as a real thing, especially in thermodynamics.
According to thermodynamic principles, the overall entropy of a system must increase over time, but positive entropy can be traded for negative entropy. This means that we (and other forms of life) must expend energy in some way that causes waste, in order to retain the predictable patterns on which knowledge relies, and over which it gives us influence.
So your observation that it can be multiplied without loss is not true. Some energy must necessarily be expended and turned into an unusable form, or no more information can be created. Those people remembering it have to eat, the paper that book is printed on had to grow and will rot, those bits stored in a computer require current flow in a circuit, that recorded medium will degrade over time and will have to be copied before it is unreadable, etc.
Then looking at knowledge one must look at the concept of knowledge: One Greek filosofer states that he knows nothing but this he knows:
"I know that I know nothing" - Socrates.
In other words: he realizes that every thing he bases his knowledge on is based on the knowledge of others. Just because one understands the line of thought dose not mean that it is true. "Why do I have two hands?" "Sow you can hold the bible you read" is a classic eksample because the question is answered with God.
An other thought is:
"I think, therefore I am" - Descartes
In this understanding knowledge is simple a product of thinking because the pure knowledge som from this. Knowledge is not defined but relative to you. This is why "God" is needed, he defines your "ground state".
Today information can be stored, meaning that a certain arrangement of electrons can lead to the storage of knowledge. To sum up: knowledge is matter of definition.
Knowledge is a label for the electrical/chemical status of particular elements of the brain. That brain state is the physical manifestation, but -since we don't typically observe any of that directly- humans generally identify with the experience of 'knowing' something instead of the physics of it.
Compare to 'happiness,' which is a similar label for what happens to the body under the influence of particular hormones. Or anger. Or a decision.
The physical manifestation of these things are arcane, and do not lend themselves to easy understanding in context of day-to-day life. People had created these labels (and spent thousands of years philosphizing and debating over them) before we had any notion of the neuro-chemistry from which they spring.
Just like software, knowledge has no real physical manifestation. What the software is stored on is physical, but software itself is more like automated concepts.
The same could be said of music: it can be written down and played, but the value of it, the reason we listen to it, exists only in our minds.
Or... you are physical, your car is physical, but what is the route you take to get to your destination? Yes, the road is physical, but your knowledge of where to go and how to get there has no real presence. More motivation or desire.
Think of knowledge as software for the human brain. It can extend your brain, increase your capacity to understand or take action. You can run it, pass it on (though the data transfer rate is slow, and there is no automatic error checking), or even delete it.
But, it has no physical manifestation. No Brain App Store, either. Wouldn't that be nice? You could buy your information, instead of having to accumulate it manually. Beware the prejudice, jealousy, hatred, and greed apps, though... they're a free download, but you pay a huge price every time you use them.
I can supply a perspective from my own (Iranian-Islamic) philosophical heritage.
Iranian Sadraian philosophers such as Allama Tabataba'i define knowledge in the broadest term as simply "the presence of a thing for another thing" where the two things are both immaterial in nature with the first thing being the knower (or "the subject" to use a fairly equivalent Western term) and the second thing being "the known" (or "the object").
This definition presumes a unity between the two since mind is believed to unite with the form of the known object once it gains knowledge thereabout, with the acquired form corresponding to the transcendental active cause of the perceived object, residing on a higher plane of existence (paralleling the Platonic theory of "Ideas").
So according to this view, every form of knowledge transcends the material world. Tabataba'i supplies a few arguments for supporting immaterial nature of knowledge. I describe only two of them that I deem to be stronger.
Humans can invoke and recall mental concepts and images from distant past which would be impossible if they were somehow "stored" in human brain or any other physical organ, as the constantly changing feature of natural things undermines any ability to preserve mental concepts through time.
The simple nature of abstract concepts negates the possibility of material basis for their preservation as a simple entity can't be ingrained in/on a composite entity.
As for transcendental origin of knowledge, after establishing its immaterial nature, he argues from the fact that neither human mind by itself nor the external world of matter can be a source for something that neither originally existed in the mind nor can come from things that are of a completely different nature, i.e. physical entities, hence the necessary existence of a higher source that embodies all forms of knowledge and is immaterial too.
Knowledge is storage of experienced examples; it is analogue to (free) energy.
Matter° (chemical substance) is simultaneously the medium to enable (e.g. physical) processes and to store the energy of these processes*. This stored energy is called the free energy A (Helmholtz energy).
Our life is analogous to the just mentioned matter°, it is the medium to enable physical and mental experiences (this is the analogue of the above processes*) and to store the results. These stored experiences are called knowledge (this is the analogue of free energy).
How these experiences are collected, how different kinds of them cooperate in this process, how they are stored, and how they are classified as examples, as inductions, as deductions, as analogies and as abductions, is described here.
All this cooperation, storage and classification of experiences occurs in our brain, in books, journals and electronic databases. — When reflecting about this, one sees that knowledge consists of stored experienced examples.