I noticed that over time, the operation manual in pdf has worsened, I consider it confusing and little detailed. A lot of useful information has disappeared. For example, the signal flow diagram in the mixer can not be found anymore.
In my opinion it should be rewritten in a simpler and better organized way.
Do you agree with me or not?
I agree. Some stuff are quite good if you know what you are looking for, but then you probably already know it. But technical manuals with the gritty technical details would be great. But I guess this is part of the Steinberg degeneration. If there is no documentation, nothing will ever fail. Very Japanese approach to get software quality.
I find the .pdf version (that I downloaded) to be sufficient enough for my purposes. I do wish they kept a link to it in the Cubase help menu like they use to. I am not a big fan of the online manual at all. I find that it is not as easily searchable like the .pdf is.
Regards 
1000% agree. The other day I wanted to link to the online manual for a post here. It took me forever to find the correct info that I already knew existed. The pdf is much easier to scroll through from page to page where the online requires clicking a link to move forward & back. The manual is organized in a way that often you can just scroll up/down a few pages when you know you are in the neighborhood of what you want.
Regarding the manual itself (and I say this as someone who in a previous life had to procure technical writing) it actually does a pretty good job for what it is, which is a reference manual. Reference manuals are meant for folks who already basically know what they are doing and just need look up some specifics.
What is really needed is an Introduction to Using Cubase manual which covers the basic concepts behind using a DAW & Cubase specifically. I think Steinberg believes they have got this covered with videos, and to an extent they do. But the problem with the videos is that the info is spread all over the place in different videos with no real sequencing of info so complexity can be introduced in a controlled manner. This is exactly the problem that a properly written intro manual can address.
So I think the OPs Manual is a pretty good reference manual, but an intro manual is very much needed. Also I think the OPs manual could go from pretty good to great if they improved the index. The actual info in the manual is well done, but finding it can be difficult or worse. Yeah we can resort to searching the pdf (and I regularly do) but that often ends up with a huge number of results to sort through. Doubling or maybe even tripling the number of entries in the index would be a huge improvement. I often find myself wondering why something as obvious as XYZ is not in the index.
Very good suggestions. 
A portion of my career was writing technical manuals for aerospace. Through customer feedback we learned that a detailed index was key (like raino mentioned). Also, we were asked to set active links in the manuals that would refer key words back to the topics in the index. These manuals were provided online in a .pdf-like format. They really worked good but, it was a huge task to get them right. We had a bunch of interns assisting.
Regards
I could not agree more (and have expressed this thought here before on more than one occasion). Steinberg is known for third rate indexes in otherwise pretty decent documentation (Wavelab documentation suffers from the same problem). The best documentation in the world does you no good if you cannot readily find the information you need in it.
Better, or more thorough, indexing of both the online and PDF manuals. To be honest I consult the online manual more often and I can usually find things by using the search box. But for that specific thing that I just.can’t.find. a deeper index would be helpful.
I agree with this.
Please provide an explanation about how this is so.
Absolutely agree - the user manual is all but useless. The producers of it clearly have given no thought whatsoever to the people who will be consulting it. It is a throwaway to which they have barely paid lip service.
Cubase is an excellent, feature packed and powerful DAW but it is very, very hard to learn and without a proper manual/user guide people will give up and the user base will dwindle.
It is very remiss and short sighted of Steinberg not to address this.
I feel very strongly that they should stop focusing on more and more features and start focusing on ACCESSIBILITY - a proper, professionally produced manual/user guide is essential - they should see to it.
I disagree. It does a pretty good job of being the type of documentation it was intended to be (as I’ve described above).
The problem is that this is not the type of manual that many folks want/need. And not providing that type of documentation is a true failure on Steinberg’s part. But that is independent of the quality of the OPS Manual.
If you need a French-to-English dictionary and I give you a French dictionary that ends up being pretty useless for you. But that doesn’t mean it is bad French dictionary, it just means it is inappropriate for your needs.
Likewise the current OPs Manual, while not great is pretty decent (compare to NI for some really sketchy documentation). Doing a pretty decent job at something you don’t need does indeed make it useless to you. But don’t make the leap from “not what I need” to “this is worthless.” They are not the same thing.
Being an old-timer, my needs are -very- different from a new user. I desire really good indexing and really good change notices because I am -only- interested in the delta… what’s been added, removed and deleted. And SB does a miserable job on this… relying far too much on time-wasting videos and putting almost ZERO effort into what I want: RECIPES. Step by step procedures.
One example, I ran across last week… I got used to the useless click track in previous versions. Now I -should- have been super jazzed with the new click track features. But frankly? If I don’t have a need for a new feature, I tend to avoid it… or continue with my stupid work-arounds. That’s on me.
However, I finally come up with a few minutes to spare and I try to figure out the new custom click patterns from the manual. And it refers to two arrows in the Transport. And I have NO idea WTF the manual means. Arrows in the -Transport-? WHERE? Finally, I noticed the little ‘+’ in the time sig and pressed it and -that- enabled the selection of custom click patterns. The wording in the manual is not terse and confusing.
They could put tool tips on the Plus icon (next to each sig). They could provide at least one RECIPE in the section on click patterns. They could say, “Press the + icon next to the time sig”, etc. Lots of =little= things that would make it a lot more fun to try things.
My point is that the manual is often so unhelpful WRT new features that I tend to carry on doing things the old way and -not- taking advantage of SB’s hard work. I suspect I’m not alone.
Completely agree! This is sorely needed.