I want to hide text which is in <span> . but why it’s not working?
.pdf {text-indent: -9999}
<p>
<a href="reader_overview.pdf" title="Open in a new Window" target="_blank"><img src="pdf.gif" alt="PDF" width="16" height="16" />
<span class="pdf">PDF 34KB, Opens in a new window</span> </a>
</p>
— http://www.w3.org/TR/CSS2/text.html
Span is, by default, inline.
The value it takes is also a length and so requires units if the value is non-zero.
There seems little point in including the information in the document if you are going to hide it though. I assume you are trying to provide information to screen reader users instead of all users … but that information is as useful to people not using screen readers.