What kind of content is it? Unfortunately, if you put ANY kind of content on the internet, someone with enough motivation, can wind up with a copy of it.
You could put it in a page, where by nature it is not exactly "downloadable" but Al is right, if it is on the internet, people will find a way to copy/download/do whatever they want with it.
If it's coming from the server to a browser then it's being downloaded. Your vendor doesn't understand how the web works.
If your actual question is "how can I stop users saving it" then it's a much more sensible question. Unfortunately, same answer.
To elaborate on the topic...
I think what you mean is to stop anyone who's downloaded & viewed the content from redistributing it. It's a common request & ultimately problematic for the reasons stated on this thread & the other that Marcus linked to.
Not mentioned so far...
One strategy is to use Digital Rights Management (DRM) which means using strong encryption, proprietary software, & often browser-incompatible formats so that only specified users can decrypt the content in order to view it, e.g. DRM enabled ebooks & PDF readers.
This also means that the content will not be compatible with some users' computers, i.e. can't or won't install DRM because it sometimes contains malware, e.g. Sony rootkit scandal: https://en.wikipedia.org/wiki/Sony_BMG_copy_protection_rootkit_scandal or is not compatible with their computer.
Another drawback is that the more steps, the more requirements, etc. that you put users through necessarily excludes a greater number of users. Whichever DRM software you choose would also need to be installed on users' tablets, phones, etc..
Also, how would you feel if you've paid to access content &/or a training/educational programme but then find out that you can't? There's the added expense of providing support to a greater number of users who have difficulties in accessing your content, some of whom will inevitably spend a lot of time on the phone, in good faith, trying to resolve their difficulties, but ultimately be left frustrated & disappointed.
As stated elsewhere in this thread, this strategy ultimately fails because someone can always find a way to circumvent DRM & copy & redistribute content. Only one person has to have to technical skills & be willing & able to do this, whereas all users of DRM-ed content at all technical skill-levels should be able to view it legitimately.
In short, I haven't heard of any feasible strategies to prevent users from redistributing digital content. Publishers don't like this because it means they have to adapt their business models to it. However, the good news is that it doesn't affect trainers & educators because that's not where the value of education & training lies, In fact, the trend is moving towards making educational & training content freely accessible, editable, & redistributable: https://en.wikipedia.org/wiki/Open_educational_resources This strategy is supported by the OECD, UNESCO, the World Bank, many universities & colleges, & many governments around the world.
I hope this makes sense.