I’m not quite following what you mean by Jupyter Notepad. However, you don’t want to be in command mode to install a package from within a Jupyter notebook because you cannot type in cells in command mode:
“When you are in command mode, you are able to edit the notebook as a whole, but not type into individual cells” - SOURCE
You want to be able to type in a notebook cell to install packages from inside a Jupyter notebook.
To install your example package from within a Jupyter notebook, you want to type the following in a normal cell still in normal editing mode:
%pip install PyPDF2
That uses the pip magic command that insures the installation occurs in the environment that is backing the kernel underlying the notebook.
For additional related information, you generally don’t need any symbol to run that command in a modern notebook because usually automagics is on by default. And by not including a symbol, the magics command gets used behind-the-scenes. What you don’t want to use in front of pip or conda is an exclamation point. Anything suggesting that is outdated and not aware of the modern magic commands for pip and conda.