Hello clement,

try this below command

sudo apt-get install python3-pypdf2

If you did install the requirements using a python virtualenv make sure you are operating with the said virtualenv activated.

If you use a python venv (as I think):

The problem is a session that is bad terminated, ODOO restart the service but not use VENV context so python3 doesn't find correct dependencies

See the correct mode to launch if venv is used:

https://github.com/odoo/odoo/issues/33479#issuecomment-1207347677

Hello,

For python3 you can use

apt-get install python3-pypdf2

or  

sudo apt install python3-pip

pip3 install -U pip

pip3 install Pypdf2

Thanks

The root cause is that your installation is lack of the list of library dependency.

step 1: run 
sudo apt install python3-dev libxml2-dev libxslt1-dev libldap2-dev libsasl2-dev \
    libtiff5-dev libjpeg8-dev libopenjp2-7-dev zlib1g-dev libfreetype6-dev \
    liblcms2-dev libwebp-dev libharfbuzz-dev libfribidi-dev libxcb1-dev libpq-dev

step 2 
$ cd to your source odoo location
 $ pip3 install setuptools wheel
$ pip3 install -r requirements.txt

you can refer here https://www.odoo.com/documentation/14.0/setup/install.html#id14

More one thing to note, to avoid conflict with other program python when install dependencies
you should create venv for your project.

Please first install virtualenv in your project directory, run

pip install virtualenv

then install project requirements, run 

pip install -r requirements.txt 

in PyCharm terminal

Cheers

https://www.odoo.com/forum/help-1/question/pypdf2-module-not-found-128668