fstl software package provides Viewer for .stl files, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install fstl
fstl is installed in your system.
Make ensure the fstl package were installed using the commands given below,
$ sudo dpkg-query -l | grep fstl *
You will get with fstl package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Graphics |
| Package | fstl |
| Version | 0.9.2-1 |
| Description | Viewer for .stl files |
| Sub Section | Graphics |