Open PDF files

• To allow saving to PDF files, install the Postscript plugin in the iv_formats.zip archive from http://www.irfanview.com/plugins.htm.
• To open PDF files, also install the AFPL Ghostscript 8.54 from http://sourceforge.net/projects/ghostscript/files/AFPL%20Ghostscript/8.54/. The GPL Ghostscript will not work (https://irfanview-forum.de/showthread.php?t=271).