Open the .plt file with 'GraphicConverter.' It's been bundled in with other Apple operating software for several years.
If not bundled; it can be located here:
http://www.lemkesoft.com/
Open the file, then print as usual, using the 'save as PDF' feature.
I use this every now and then for HP files or other plot files for converting them to PDFs.
Hope this helps.
Mark
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Mac G5 Dual 2.3 Ghz, 1.5 Gb, Mac OSX.4.11, ArchiCAD 12 2782 USA Full + 11, 10, 9 & 7
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Mark R. Wallace AIAa
Mark R. Wallace AIA
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MacBook 2.53 Ghz, Intel Core i5, 8 Gb,
Mac OSX (Sierra 10.12.6,
ArchiCAD 22 USA Full, +21, & 20.