I installed Seafile on my home server (running Ubuntu 16.04) by following the official instructions at manual.seafile.com, and I use a separate, non-privileged user account to 1.store the files (seafile-server is contained in that user’s home directory: /home/user/seafile-server-6.3) and 2.launch the service through systemd.
Since I only install the required pip libraries (e.g. pillow, moviepy, django-pylibmc) because of Seafile, and in order to contain things even further, could I install such libraries to the user home directory using ‘pip install --user’ instead of installing system-wide?
Would Seafile (or even systemd) have any difficulty detecting and running those libraries, because of them being installed only for the user that runs the Seafile service?
You mustn’t install them, they’re included in the seafile package. If you do it, you’ll get errors.
I installed them because the official instructions explicitly tell to issue the ‘pip install’ commands:
1 - pillow & moviepy: https://manual.seafile.com/deploy/using_mysql.html
2 - pylibmc & django-pylibmc: https://manual.seafile.com/deploy/add_memcached.html
Is the official documentation outdated?
It’s a mess and the devs don’t seem to care much about it for years. 
No, the installation is correct.
Regarding your initial question: I don’t know how exactly it works. As long as the libraries can be found it works. Most likely there is some path. This is a general python “thing”.
I don’t know where you got that information from it is neither documented nor could I find e.g. pillow in my seafile-server-latest folder (while I could find it on my system at /usr/local/lib/python2.7/dist-packages/imageio/plugins/pillow.py - installed via pip).
This is also not true. The manual installation always worked like a charm.
I am talking about the order/overview and usability of the manual. This is a mess and information is spread and not written on a logical prefer that helps people to get started easily.
There is just no guideline.
But it’s totally unrelated to this topic. The user wants to do something beyond that what is documented. That’s no issue and most likely possible but not something which is missing in the manual.
The documentation is correct just bionade24 told something wrong, here. No reason for you to spam something unrelated, because as you should have seen the user found the documentation and was able to follow it.
As, according to @shoeper, this is not documented – and I couldn’t find a similar question anywhere, I’ll simply perform a test.
Thanks.
UPDATE:
I performed the tests, and it works well. Didn’t have to install the pip modules system-wide; installing them as the user that runs the seafile-server process using the command ‘pip install --user’ worked.
Seafile was installed from scratch on Ubuntu 18.04.