Hi graham alsltd,
As Acrobat is crashing when trying to open a second pdf, could you try rebooting the machine once if you haven't already and check? Does that help? If not, check if an update is available for Acrobat using "Check for updates" option under "Help" menu, reboot the machine after installing update and try again. You may also download updates from here: Adobe software and product updates
If issue still persists, try repairing Acrobat using "Repair installation" option under Help menu, reboot the machine after repair and check.
Is there any error message when Acrobat crashes?
Also, let us know dot version of Acrobat installed on the machine? You may refer to the steps given in this link on how to check the version in Acrobat: Identify the product and its version for Acrobat and Reader DC
Let us know if you need any help.