I have several diagrams that I attached to this post. Does anyone know what software was used to make them? I attached the diagrams as PNG files but in PDF files that I have, they are in vector graphics.
The presence of "Corel Office Document" in the properties of one of the PDF documents suggests that Corel Draw was used to produce them.
Electronic symbols libraries are available for Corel Draw.
The presence of "Acrobat Distiller 4.05 for Windows" might mean that Acrobat Distiller was then used to shrink some of the PDFs and/or make them more generally compatible with PDF readers.