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

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

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.