qpdf: add merge files example (#3120)

pages/common/qpdf.md

@@ -7,7 +7,11 @@
 
 `qpdf --empty --pages {{input.pdf}} {{1-3,5,6-10}} -- {{output.pdf}}`
 
-- Merge (concatenate) a list of PDF files and save the result as another one:
+- Merge (concatenate) all the pages of a list of PDF files and save the result as a new PDF:
+
+`qpdf --empty --pages {{file1.pdf}} {{file2.pdf}} {{file3.pdf}} -- {{output.pdf}}`
+
+- Merge (concatenate) given pages from a list of PDF files and save the result as a new PDF:
 
 `qpdf --empty --pages {{file1.pdf}} {{1,6-8}} --pages {{file2.pdf}} {{3,4,5}} -- {{output.pdf}}`