In fact i've got a list of numbers enclosed in <pdf> tags and i want to set the variable "present" with O if the number i search for is present
<xsl:variable name="present">
<xsl:for-each select="/liste/id/compte/pdf">
<xsl:if test="/liste/id/compte/pdf=$ID">O</xsl:if>
</xsl:for-each>
</xsl:variable>
<xsl:value-of select="$present"/>

The test
    test="/liste/id/compte/pdf=$ID"
will succeed if there is at least one of the pdf elements
with a content equal to value of $ID. This seems to be
exactly what you ask for. Try:

   <xsl:variable name="present">
      <xsl:if test="/liste/id/compte/pdf=$ID">O</xsl:if>
   </xsl:variable>