If the following plot settings are always the same then you can load PlotMe.lsp (revised attached) at startup and it'll always be available to run:
paper size = ISO full bleed A3 (420.00 x 297.00 MM), plot area = Extents; Scale = 1=1, plot style table = acad.ctb
Otherwise you may end up having different versions saved with a different lisp file name which you can then load them all at startup.
; PlotMe generates pdf with DWG To PDF.pc3
; OP: https://forums.autodesk.com/t5/autocad-forum/how-to-set-the-default-plot-location-as-the-dwg-location-when/m-p/13742012#M352292
(defun c:PlotMe (/ cmdecho ctab filedia pdfname pltare pltctb pltdev pltlwt pltoff pltori pltppr pltscl pltsty pltuni pltupd)
(vl-load-com)
(setq cmdecho (getvar "cmdecho") ; save current settings
filedia (getvar "filedia")
ctab (getvar "ctab")
; pdfname (strcat (getvar "dwgprefix") (vl-filename-base (getvar "dwgname")) "-" ctab ".PDF") ; pdf with layout name appended
pdfname (strcat (getvar "dwgprefix") (vl-filename-base (getvar "dwgname")) ".PDF") ; pdf name
)
(setvar "cmdecho" 0) ; suppress command echo
(setvar "filedia" 0) ; suppress file selection window
; begin command line plot sequence
(command "_.-PLOT"
"_Yes" ; Detailed plot configuration? [Yes/No]
ctab ; Enter a layout name or [?] ; or just return "" to accept current layout
"DWG To PDF.pc3" ; Enter an output device name or [?] ; "AutoCAD PDF (High Quality Print).pc3" "AutoCAD PDF (General Documentation).pc3" "AutoCAD PDF (High Quality Print).pc3" "AutoCAD PDF (Smallest File).pc3" "AutoCAD PDF (Web and Mobile).pc3"
"ISO full bleed A3 (420.00 x 297.00 MM)" ; Enter paper size or [?] ; unique for selected printer device
"_Millimeters" ; Enter paper units [Inches/Millimeters] ; "_Inches"
"_Landscape" ; Enter drawing orientation [Portrait/Landscape] ; "_Portrait"
"_No" ; Plot upside down? [Yes/No]
"_E" ; Enter plot area [Display/Extents/Layout/View/Window] "_D" "_L" "_W" "_V"
"1=1" ; Enter plot scale (Plotted Inches=Drawing Units) or [Fit] ; "1:1"
"0,0" ; Enter plot offset (x,y) <0.00,0.00>: ; "0,0"
"_Yes" ; Plot with plot styles? [Yes/No]
"acad.ctb" ; Enter plot style table name or [?] (enter . for none)
"_Yes" ; Plot with lineweights? [Yes/No]
)
(if (= 1 (getvar "tilemode")) ; chk space
(command ; then model
"_A" ; Enter shade plot setting [As displayed/legacy Wireframe/legacy Hidden/Visual styles/Rendered] <As displayed>:
)
(command ; else layout
"_No" ; Scale lineweights with plot scale? [Yes/No]
"_No" ; Plot paper space first? [Yes/No]
"_No" ; Hide paperspace objects? [Yes/No]
)
)
(command
; "_Yes" ; Write the plot to a file [Yes/No] ; extra prompt may occur for custom pc3
pdfname
"_Yes" ; Save changes to page setup [Yes/No]
"_Yes" ; Proceed with plot [Yes/No]
)
(setvar "filedia" filedia) ; restore original settings
(setvar "cmdecho" cmdecho)
(princ) ; clean exit
)
Note: lines beginning with a semicolon ";" in the lisp code is not executed.