Here's a quick example where I get the values from my multi-select choice column then join them together.
The SharePoint item I'm retrieving is ID = 4 in the following list. I'll retrieve all the Country values.

See full flow below. I'll go into each action.

Get item retrieves the item from the list.

Select takes in the Countries from Get item, and uses the following expression to extract the values. Note that I'm also using Text mode (see arrow on screenshot below).
item()?['Value']

Join takes the output from Select and joins each of the items using ", ".

This would give the following output.

If you didn't want to use a Join action (as above) and just use an expression to join them, you could do the following.
join(body('Select'), ', ')
Once you have the values joined, you can add them to your PDF.