pstoedit+ghostscript
1.download the last version pstoedit and ghostscript
2.install pstoedit and ghostscript
3 run command pstoedit -f dxf in.pdf out.dxf(make sure pstoedit can find the path of gswin32c.exe)

pstoedit+gsview
1.download the last version pstoedit and gsview
2.install pstoedit and gsview
3. use pstoedit as a plugin of gsview

pstoedit is a tool converting PostScript and PDF files into various other formats suported by different drawing editors. As a prerequesite it needs GhostScript to be installed (binary installation is sufficient).

Currently pstoedit can generate the following major formats:

• Tgif .obj format (for tgif version >= 3)
• .fig format for xfig
• pdf – Adobe’s Portable Document Format
• gnuplot format
• Flattened PostScript (with or without Bezier curves)
• DXF – CAD exchange format
• LWO – LightWave 3D
• RIB – RenderMan
• RPL – Real3D
• Java 1 or Java 2 applet
• Idraw format (in fact a special form of EPS that idraw can read)
• Tcl/Tk
• HPGL
• AI (Adobe Illustrator) (based on ps2ai.ps – not a real
  pstoedit driver – see notes below and manual)
• Windows Meta Files (WMF) (Windows  only)
• Enhanced Windows Meta Files (EMF) (Windows, but also Linux/Unix if
  libemf is available)
• OS/2 meta files (OS/2 only)
• PIC format for troff/groff
• MetaPost format for usage with TeX/LaTeX
• LaTeX2e picture
• Kontour
• GNU Metafile (plotutils / libplot)
• Skencil ( http://www.skencil.org)
• Mathematica
• via ImageMagick to any format supported by ImageMagick
• SWF
• CNC G code
• VTK files for ParaView and similar visualization tools

From above cheat, we know pstoedit supports generate DXF, but it needs the help of Ghostscript or GsView

Ghostscript is the name of a set of software that provides:
* An interpreter for the PostScript (TM) language and the Adobe Portable Document Format (PDF — sometimes confused with Acrobat, Adobe’s PDF browser/editor product), and
* A set of C procedures (the Ghostscript library) that implement the graphics and filtering (data compression / decompression / conversion) capabilities that appear as primitive operations in the PostScript language and in PDF.

reference,
GPL Ghostscript
GSview
pstoedit

A friend gave me a PDF document with Certificate encryption, he bought from a vendor. The PDF even does not allow copy text, so he wanted me to crack it. After effort, I removed the encryption. And I also developed a software to do this kind job without need Adobe Acrobat, but I am not sure if it is legal or not, so I can not release it until I know it is legal.

BTW, Digital Signature vs (Certificate) Encryption
A digital signature can guarantee the integrity of the message and the identity of its sender. Encryption can prevent someone from being able to read an intercepted message.

Update: just noticed that LiveCycle Workbench ES has the feature, Remove PDF Certificate Encryption

Returns an unsecured PDF document when given a PDF document encrypted with a certificate. After certificate encryption security is removed from a PDF document, it is no longer secure.

Reference,
Remove PDF Certificate Encryption

I do not know what’s wrong with my Google Chrome Frame, when I tried to sign in one of my Gmail account(another account does has this problem), I always got the alert as the following screen shot.

and when I click OK, I got sign out.

Alert https://mail.google.com
You have been signed out this account.
This may have happened automatically because another user signed in from the same browser. To continue using this accout, you will need to sign in again.
This is done to protect your account and to ensure the privacy of your information.

I am sure I have signed out another account and try this account again and again, but without any success.