please double check if the url used to open the pdf file from custom url scheme is correct.
I've made some test with my custom url scheme "radaee://", and these are the results:
I noticed that if I use something like "radaee://http://my_destination_url/my_pdf.pdf", I have to remove radaee://.
This is not enough, cause I noticed that with the url above, ":" char is removed from "http://" string.
I suggest to set your custom url in this way:
than replace your custom url scheme with http or https before your openURL method.
With the result string I'm able to open the pdf using ShowReader method.