You can use WebClient.DownloadData
,
which has a default return value of a byte array. e.g.
byte[] bytes = myClient.DownloadData("https://www.member-data.com/files/hb/c8955fc4d6160ec0fd87f4879c6496d3.pdf");
Also, this assumes that you want the actual file in a byte array, not the content (text) of the PDF. That's a whole other ball of wax.
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Free .NET PDF
library supporting convert and other operation of PDF.