I have this link viewdocument.php?a=http%3A%2F%2Fwww.iesalfonsox.com%2Fimages%2FLIBROS_ESO_13-14.pdf in (for example) index.php and I want to put only the URL of the document of the link.
Example:
Link: http://www.myweb.com/viewdocument.php?a=some.pdf
On the HTML: You're viewing "Some.pdf".
Do you understand? Convert "viewdocument.php?a=some.pdf" to "some.pdf".
If this is the current URL being accessed on your site, Php provide globals and you can access it using the $_GET variable:
so for your example you will use this:
$file = $_GET['a']; // will return some.pdf
The other case will be for parsing a url from a string, for which php provides a function:
Follow the link to see examples