Unlike PDF files which are created to be viewed PLT files are created to be printed or plotted to a specific printer or plotter. PLT is not even a format, it's merely a file extension that programs use to specify that it is a plot ready file. AcroPlot and AcroPlot Repro can properly read the vast majority of HPGL and HPGL/2 "PLT" files out there. HPGL and HPGL/2 was a plotter format designed and published by HP back in the 80's and 90's. There hasn't been an official update to the published format for over a decade that we know of.
Why won't AcroPlot or AcroPlot Repro convert my PLT file?
Most likely the "PLT" file is not really an HPGL or HPGL/2 format. AutoCAD will use the PLT extension for any plotter driver that you choose the plot to file with. This may include a Postscript driver, a PCL driver, or even a driver for say an Epson inkjet printer that might use a proprietary program. While most large format plotters worked with the HPGL format many also shipped a postscript version of their drivers also.
As such we cannot work with these file types. If it is a know file type such as PDF or Postscript (if you have Ghostscript installed) we will be able to determine that and convert it anyways.
We have also rarely ran into files which are basically an HPGL or HPGL/2 format file with specific extensions to the format added by a plotter manufacturer. Since the official format has not been updated in over a decade manufacturers have added their own programming codes to it to support specific features for their plotter. If we do convert it but something is not correct in the results please upload the problem file to our support desk so we can see if we can support their changes.
Why are my PLT files rotated different directions?
Again since the PLT files were created to plot to a specific plotter they might be rotated based on a the width of the plotter. If a 24x36" drawing was plotted to a 24" wide plotter it would have had to be rotated 90 degrees. But to a 36" wide plotter it most likely would not have been rotated.
We do have a feature on the Pro Options Tab in AcroPlot Pro which will allow you to "Force to landscape rotation". There is also a "Force to landscape default rotation" (Landscape default rotation in some versions) of either 90 or 270 degrees to control if you want portrait drawings rotated CW or CCW when they are rotated to landscape. Unfortunately this is mainly a trial and error type process with your specific files.
Also to make matter worse AutoCAD has always allowed you to plot the file upside down also. This was sometimes used by specific users to make their drawing stand out against all the other users drawings in the stack at times. If you run into Landscape drawings which are upside down you can set the "Rotate landscape files 180 degrees" to On.
You can also use the AcroPlot Pro preview to look at each PLT file and manually rotate it prior to converting it.