PDF

You may have found a solution already but here is what I know regarding plotting high resolution images out from AutoCAD.   The stuff behind a PDF is essentially an image in most cases and for the purpose you describe it is.



Plot to a raster device (TIFF, JPG, etc.)  The device for these is delivered with AutoCAD.  In the Add-a-plotter wizard scroll dow the left (device) window to "Raster Devices" and select either JPG or TIFF.  Use JPG for full color and use TIFF for Black and White.  Once you have the plotter added, customize it by adding the paper size you need (use the plot dialog).  Raster plotters do not have any standard paper sizes that actually correspond to real-world paper sizes so they must be added.  The reason that there are none is that each one must be resolution dependent.  A separate custom paper size is required for every paper size and resolution that you want to plot to.  For each of them, but especially for the very large 4'x8' output you desire, you will need to determine the absolute minimum resolution that still meets your "high-resolution" requirement.  There are a couple of reasons for this.  1) the file size increases exponentially with increases in resolution.  2) most image editing/publishing software has trouble with images that have more than 10,000 pixels in either dimension.  To stay under 10,000 pixels in 96" your output dpi needs to be 104 dpi or less.  For an outdoor sign that should be fine.  For 104 dpi on your custom 48"x96" paper size your dimensions need to be 9984 pixels x 4992 pixels.  (You can round it to an even 100 and make the pixel dimensions 9600x4800 if you like).  Plot your image to file with these parameters and it should be relatively small, problem free with Adobe and any other image software and very sharp for outdoor reading distances.  If Adobe or other image software can handle it, you can increase the dpi and pixel dimensions to get an even higher resolution (I use 288dpi for my ANSI-D plots 'cause it is a multiple of 72).



 NOTE: you will need to copy whatever pen table you are using (if you use pen tables) and increase the pen sizes for resolutions higher than 100dpi by a factor of [higher resolution/100] because AutoCAD assumes all raster plots are 100dpi regardless of the dpi value you specify in the plot dialog.  Pen sizes don't scale with your dpi resolution automatically.  Each dpi requires a separate pen table to maintain standard pen thicknesses across all resolutions.  There is a trick to avoid needing multiple pen tables but it is beyond the scope of this discussion.



Your can convert your images to single or multipage PDF for free and very nicely using a free app that has massive acclaim on the internet.  IrfanView is its name, Google it and visit the IrfanView site.  Be sure to download the full addon pack that includes the PDF app that another kind programmer has made available for creating and editing PDF's.  If you have any Adobe greater than Reader you can probably make the PDF using Adobe just as easily.



A final caveat, raster output from AutoCAD is always unitless.  This means that the image has no recorded dpi resolution.  This is a problem even for Adobe.  Adobe will assume the resolution of AutoCAD raster plots is always 72dpi.  It will still have the pixel dimensions you specified but a 104dpi plot on 96"x48" paper will result in a paper size of 138.67" x 72" when Adobe converts it to a PDF using the default 72dpi.   This is not a problem for IrfanView.  IrfanView will easily allow you to specify and respecify image resolution.  Just use IrfanView to set the dpi to match the dpi you specified in AutoCAD, convert it to PDF and it will be the exact size and scale you intended. This is a critical step if your output must be to an exact scale.