In addition to the posts above.
You can redirect the output of the show tech-supp to either FTP, TFTP, flash .....
show tech-support | redirect tftp://192.168.100.136/myrouter_tech
Or you can redirect the show tech support file to the router flash and after that you can copy it from the flash to the tftp server.
Try a "show tech-support page", but difficult to stop once started.
Or, as noted already use the redirect and send to flash. But, then read with "more":
show tech-support | redirect flash:show-tech
If you are using PuTTy, Go to settings-> Select Logging under "Session"
then select "All session output and in the "Log file name" create a new txt file or browse to a folder wherever you want to save the output. Start your session, run few commands to verify it copies the output to your file.