Two similar routes that we use both use the Ghostscript/Ghostgum combination of open source, freely available software. (They run on both Windows and Linux.)
Method 1: Export a tiff of the plot and then use ghostgum to convert the tiff to pdf.
better though is:
a) install a Postscript driver on your computer. HP has heaps of PS plotter drivers that can be used. Pick one that supports large pages - virtual paper is fairly cheap
make sure the plotter/printer driver is installed to default to "print to file".
c) you may need to create a custom paper size - details of this vary with the driver.
d) plot to this PS driver using your custom paper size.
e) run Ghostgum to convert PS to PDF.