QGIS 3.0 migration - unable to install PyPDF2

I am migrating my Plugin to QGIS 3.0. Currently a bit stuck trying to install PyPDF2. I have been trying different things for the last couple of hours or more, without success.

I have got another version of Python installed (3.5, which I use for other things and 2.7…partly – I uninstalled it via Windows today although the folder is still there at C:Python27. Probably I have to manually delete the remainder) Anyway I believe it should be quite possible to run more than one version. I have done it before.

QGIS uses Python 3.6, here: C:Program FilesQGIS 3.0appsPython36

So in the standard scenario I would install PyPDF2 from OSGeo4W Shell. I run OSGeo4W Shell and type o-help which shows a list of available modules including pip3 (although not pip). I presume I should use pip3 to install PyPDF2. So I type this command:

pip3 install PyPDF2

I get the familiar message 'Fatal error in launcher: Unable to create process using '"''

I have trawled the forums for solutions to this but the ones I have tried have not worked for me. It sounds as if this should be fixed by upgrading pip, from what I understand(?)

If I run pip3 install --upgrade pip3 it doesn’t work either. Same error, as might be expected

I have tried to run pip / pip3 from a command window opened from the path C:Program FilesQGIS 3.0appsPython36Scripts

If I do ./pip3 install PyPDF2 from a Powershell window there I get the same error again.

If I go up one level to the Python36 folder and run python -m pip3 install PyPDF2 I get the error No module named pip3

And so and so forth.

In an effort to make sure I am using the correct (QGIS) version of Python I added the ‘QGIS 3.0appsPython36’ folder to the PYTHON_HOME variable and added %PYTHON_HOME% to the Path variable.

How can I install PyPDF2??

C:Users[user]AppDataLocalProgramsPythonPython35Libsite-packages

C:Program FilesQGIS 3.0appsPython36Libsite-packages