If you routinely want to view a large number of posts on one page, after you’ve logged in click on User CP (control panel) at the top left of the screen.
scroll down to Edit Options - click
Then scroll down to the Thread display options (It is set to Use Forum Default)
change the number of posts to show per page to the maximum permitted (40).
Then scroll down to the bottom of the page and click Save changes.
If you only want to have a large number of posts occasionally, for the purpose of printing, then when you’re in the thread you want to print click on Thread tools (near the top right of the thread) and from the dropdown menu choose Show printable version. If it’s a long thread, there will be an option to show 40 posts on the same page. (This option doesn't appear if thread is very short.) It mightn’t get everything onto the same page but it’ll help.