(Poppler was added as a cygwin package about a year after I wrote this answer. Now it installs like any other package.)
Poppler is not a cygwin package. There is no "command" to install it. You need to unpack and build it yourself. (Poppler has two files that you need to download.)
The download is a .tar.gz file. Unzip it, unzip it and go to its directory.
$ gunzip filename $ tar -xvf filename $ cd poppler-0.16.5
Now find the files with the names "README" and "INSTALL". Read them and follow their instructions.
If you are missing something, download it from Cygwin and install it this way. What you may need if you did not create the program from the source before turning it on
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