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+You can join multiple images to pdf using `convert`:
+
+    convert *.png out.pdf
+
+If you get an error like 
+
+    convert-im6.q16: attempt to perform an operation not allowed by the security policy `PDF' @ error/constitute.c/IsCoderAuthorized/408.
+
+follow the steps below:
+
+    sudo vim /etc/ImageMagick-6/policy.xml
+
+and replace the line 
+
+    <policy domain="coder" rights="none" pattern="PDF" />
+
+with
+
+    <policy domain="coder" rights="read|write" pattern="PDF" />
+
+
+[source](https://askubuntu.com/questions/1081895/trouble-with-batch-conversion-of-png-to-pdf-using-convert)