The printer is most likely configured to use its own fonts. If a report is printed using a font which the printer does not recognise, it may print random characters/symbols in place of text. When previewing the reports in DARWIN, they should look OK.
This can be solved by configuring the printer either to download unknown fonts (Softfonts) from the PC which is printing the report, or to print as graphics rather than text. The latter option may cause printing to take a bit longer, but only experimentation will tell you how long and whether this will be noticeable.
Printers can be configured from the Printers applet in the Windows Control Panel. If you right-click on a printer and select Properties, you can change the behaviour of the printer. Unfortunately the layout and contents of this screen are partially dictated by the printer drivers and so will be different for each type of printer.
The setting required should be available by clicking on the "Printing Preferences" button, then clicking on the "Advanced" button. There should be a Graphic/Font option, which is currently set to "Device Font". This needs to be changed to "Download Softfont" or "Print as Graphics". If there are no font options in this view, then consult the printer manual for documentation on configuring it to print unknown fonts.