Which is your pdf reader? Assuming Adobe Reader, see:
http://www.adobe.com/cfusion/knowledgebase/index.cfm?id=328233
"PDF pages don't appear in web browser window (Acrobat 7.0 and 3D, Adobe
Reader 7.0-8.0, and Acrobat 8)".

You might find this report useful.

       National Cyber Alert System

  Technical Cyber Security Alert TA08-043A

Adobe Reader and Acrobat Vulnerabilities

  Original release date: February 12, 2008
  Last revised: --
  Source: US-CERT

Systems Affected

    * Adobe Reader version 8.1.1 and earlier
    * Adobe  Acrobat  Professional,  3D, and Standard versions 8.1.1 and
      earlier

Overview

  Adobe  has  released  Security  advisory APSA08-01 to address multiple
  vulnerabilities affecting Adobe Reader and Acrobat. The most severe of
  these  vulnerabilities  could  allow  a  remote  attacker  to  execute
  arbitrary code.

I. Description

  Adobe    Security   advisory   APSA08-01   addresses   a   number   of
  vulnerabilities  affecting  the  Adobe  Acrobat  family  of  products,
  including  Adobe  Reader.  Acrobat  versions  8.1.1  and  earlier  are
  affected.  Further  details are available in the US-CERT Vulnerability
  Notes Database.

  An  attacker  could exploit these vulnerabilities by convincing a user
  to load a specially crafted Adobe Portable Document Format (PDF) file.
  Acrobat  integrates with popular web browsers, and visiting a web site
  is usually sufficient to cause Acrobat to load PDF content.

  At least one of these vulnerabilities is being actively exploited. The
  SANS Internet Storm Center Handler's Diary contains more information.

II. Impact

  The  impacts  of  these vulnerabilities vary. The most severe of these
  vulnerabilities allows a remote attacker to execute arbitrary code.

III. Solution

Upgrade

  Upgrade  Adobe  Reader  or  Acrobat  to version 8.1.2 according to the
  information in Adobe Security advisory APSA08-01.

Disable web browser display for PDF documents

  Preventing  PDF  documents  from  opening  inside  a  web  browser may
  mitigate  this  vulnerability.  Applying  the  following workaround in
  conjunction  with  upgrading  may prevent similar vulnerabilities from
  being automatically exploited.

  To  prevent  PDF  documents  from  automatically being opened in a web
  browser with Acrobat or Reader:
   1. Open Adobe Acrobat or Adobe Reader.
   2. Open the Edit menu.
   3. Choose the Preferences option.
   4. Choose the Internet section.
   5. De-select the "Display PDF in browser" check box.

Disable automatic opening of PDF documents in Microsoft Internet Explorer

  To  disable  automatic  opening  of  PDF  files  in Microsoft Internet
  Explorer  (IE),  a  second step is required. To configure IE to prompt
  before  opening  a  PDF  file,  disable  the  "Display PDF in browser"
  feature  (as  described  above) and then make the following changes to
  the Windows registry:

      Windows Registry Editor Version 5.00

      [HKEY_CLASSES_ROOT\AcroExch.Document.7]
      "EditFlags"=hex:00,00,00,00

Disable JavaScript in Adobe Reader and Acrobat

  Disabling  JavaScript  in  Adobe  Reader  and Acrobat may prevent this
  vulnerability  from being exploited. In Acrobat Reader, JavaScript can
  be  disabled  in  the General preferences dialog (Edit --> Preferences
  --> JavaScript, de-select Enable Acrobat JavaScript).

IV. References

    * US-CERT  Vulnerability Notes for Adobe Security advisory APSA08-01
      - <http://www.kb.cert.org/vuls/byid?searchview&query=APSA08-01>

    * Securing Your Web Browser -
      <http://www.us-cert.gov/reading_room/securing_browser/>

    * Adobe Security Advisory APSA08-01 -
      <http://www.adobe.com/support/security/advisories/apsa08-01.html>

    * Adobe Reader 8.1.2 Release Notes -
      <http://www.adobe.com/go/kb403079>

    * SANS Internet Storm Center Handler's Diary -
      <http://isc.sans.org/diary.html?storyid=3958>

    * Configuring Windows Explorer - Registry EditFlags -
      <http://mc-computing.com/WinExplorer/WinExplorerEditFlags.htm>

    * Internet Explorer Opens .exe Files Instead of Downloading Them -
      <http://support.microsoft.com/kb/140991>

    * Office Documents opening in IE -
      <http://blogs.msdn.com/omars/archive/2004/04/29/123181.aspx>

____________________________________________________________________

  The most recent version of this document can be found at:

    <http://www.us-cert.gov/cas/techalerts/TA08-043A.html>
____________________________________________________________________

  Feedback can be directed to US-CERT Technical Staff. Please send
  email to <cert@cert.org> with "TA08-043A Feedback VU#666281" in the
  subject.
____________________________________________________________________

  For instructions on subscribing to or unsubscribing from this
  mailing list, visit <http://www.us-cert.gov/cas/signup.html>.
____________________________________________________________________

  Produced 2008 by US-CERT, a government organization.

  Terms of use:

    <http://www.us-cert.gov/legal.html>
____________________________________________________________________

Revision History

  February 12, 2008: Initial release