pdfjoin: add page (#2040)

pages/common/pdfjoin.md

@@ -0,0 +1,15 @@
+# pdfjoin
+
+> PDF merging utility.
+
+- Merge two PDFs:
+
+`pdfjoin {{file1}} {{file2}} --outfile {{output_file}}`
+
+- Save pages 3 to 5 followed by page 1 to a new PDF:
+
+`pdfjoin {{file 3-5,1}} --outfile {{output_file}}`
+
+- Merge subranges from two PDFs:
+
+`pdfjoin {{file1 3-5,1}} {{file2 4-6}} --outfile {{output_file}}`