Thank you for showing interest in GroupDocs.Conversion API.
Yes DWG/DXF files are supported and you can convert them into PDF and Image files. Please click here for more details regarding supported document formats.
Your second question regarding PLT files, we are sorry currently this file format is not supported in our document conversion api, however we will forward your requirement of PLT file format for feasibility to our product team.
If you will need any help or you will have any other questions regarding GroupDocs.Conversion API please feel free to ask.