HP laserjet5p and Scanjet4370 to a 2012 iMac with OS high Sierra. HP site has no drivers available

The HP LaserJet 5P is mentioned in the following article. To begin with, could you check whether the printer software has been automatically installed?

Printer and scanner software for macOS High Sierra, Sierra, El Capitan, Yosemite, and Mavericks - Apple Support

The LaserJet 5P model is a parallel-port printer. It should be possible to connect it via a print server (Ethernet) or through a USB-to-parallel adapter, but an appropriate printer driver would be needed.

Hello again,

I really have to guess here. The code produced appears to indicate something related to PostScript. Could it be that you have chosen a driver for the HP LaserJet 5MP (please see the reply by _laure_ under Re: HP Laserjet 5P driver issues)? If so, is there possibly an HP LaserJet Series PCL 4/5 (or similar) to be found via a Use menu (Other... ?) instead when you are trying to add a printer?

Also, the following post could perhaps be of interest:

Re: HP Laserjet 5P driver issues

>I tried the site and downloaded the driver

Hi patifromlakewood,

In order to avoid any misunderstanding (unless you have solved this matter already), where did you download the driver? Via Apple's HP Printer Drivers v5.1 for OS X download page (Post Date: Oct 24, 2017), or was it possibly something from http://gimp-print.sourceforge.net/MacOSX.php? What was the name of the file? Anything similar to the file name shown in the reply by Old Toad?

BTW, the information available is not always easy to interpret:

According to the aforementioned Apple article Printer and scanner software for macOS High Sierra, Sierra, El Capitan, Yosemite, and Mavericks - Apple Support (Published Date: Nov 4, 2017): "Your Mac automatically installs third-party software (drivers) for the following printers and scanners."

According to a note in an HP support article https://support.hp.com/us-en/product/hp-laserjet-5p-mp-printer-series/25481/docu ment/c04510394 (accessed today): "Since October 2016, HP printer software packages are no longer delivered through Apple Software Update."

my Mac wouldn't let me open it as it was not from Apple.

Control (right) - click on the driver installer and select Open from the contextual menu: