I wouldn't replace :// with %3A%2F%2F, there's no reason to do that, it might even cause trouble in some instances. You only need to URL escape characters which are invalid for URLs, the protocol specification by :// and forward slashes are allowed in general, except in the arguments of HTTP-GET data.
If your href attribute doesn't start with a protocol specification (like
http://, file://, ftp:// ...), its value will get interpreted as a reference to a local file, as a path relative to the location of the current document.
Code:
Text/http://hebrewbooks.org/pdfpager.aspx
therefore references a file
Code:
http://hebrewbooks.org/pdfpager.aspx
in the
subdirectory of the directory in which the HTML file is located, which obviously doesn't make any sense, because you're actually trying to create a HTTP link to an external resource. Solution: just remove the 'Text' part, you might have copied and pasted it in error.