docs(viewer): document the PDF context-menu export

The context menu now offers PDF alongside CSV and Excel, and the three are
"what you see" exports rather than the backend download, so say what that
means and what the PDF option does not do (5000 row cap, monospace, Western
European characters only).

docs/dcu-tableviewer.md

@@ -49,7 +49,9 @@ The Download button gives several options for obtaining the current view of data
 
 Note - if the table is registered in Data Controller as being TXTEMPORAL (SCD2) then the download option will prefilter for the _current_ records and removes the valid from / valid to variables.  This makes the CSV suitable for DC file upload, if desired.
 
-Note that all the above items are exported from backend.  There is also a _frontend_ context menu that can be used in the VIEW screen (right click menu) for quick export into CSV or Excel formats.   
+Note that all the above items are exported from backend.  There is also a _frontend_ context menu that can be used in the VIEW screen (right click menu) for quick export into CSV, Excel and PDF formats.  These frontend exports are "what you see": they cover the grid as it is displayed (the current selection if you have one, otherwise the whole table, with hidden columns omitted), so they honour your on-screen filters and labels but are not a substitute for the backend download when you need the full table.
+
+The PDF option produces a paginated A4 landscape document with the column headers repeated on every page.  It is limited to the first 5,000 rows, uses a monospace font, and covers Western European characters only - anything outside that character set is shown as `?`.  For a typeset report with your company branding, use the backend download instead.
 
 ### Web Query URL