Installing pdftoppm and/or Ghostscript

Some options of pdfCropMargins depend on either the Ghostscript program or the pdftoppm program being installed (and locatable) on the system.

pdftoppm

Linux: The pdftoppm program is standard in most Linux distributions and is easy to install in Cygwin. It is currently part of the Poppler PDF tools, so that package should be installed on Linux and Cygwin if the pdftoppm command is not available. On Ubuntu the command is:
```
sudo apt install poppler-utils
```
Windows: In Windows pdftoppm is not as easy to install, but a collection of PDF tools found here includes pdftoppm. That version is bundled with the software and will be used as a fallback on Windows if neither Ghostscript nor the system pdftoppm program can be found.

Linux: Ghostscript is in the repos of most Linux distributions and is easy to install on Windows and in Cygwin. On Ubuntu the command is:
```
sudo apt install ghostscript
```
Windows: The Windows install page is located here; the non-commercial GPL version on that page should work fine for most people. Add the directory of the executable gswin64c.exe (or the 32 bit version if you installed that) to your Windows system path so it is discoverable (and runnable from the command shell). On Windows 10 the place to go is:
```
Start -> Control Panel -> System and security -> System -> Advanced system settings
```
Now click "Environment Variables" and then double click on the user variable Path. Click "New" and browse to the directory to add (something like C:\Program Files\gs\gs9.27\bin). Restart your command shell for the change to be recognized.