Try setting the command to a variable, repeat it and try directly from the executable file, not from PHP. This may give a readable error.
Also, try putting the url in quotes, I just tried the following command and it worked perfectly
wkhtmltopdf.exe "https://www.google.com/search?hl=fi&safe=off&biw=1195&bih=732&site=imghp&tbm=isch&sa=1&q=test&oq=test" test.pdf
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