Set the key length to 40 when encrypting pdf files

Key length is sometimes used to measure the relative strength of the encryption algorithm (i.e., the longer the key length, the more difficult the key is to decode). Encryption can be describes as 40-bit &128-bit in terms of key length. The 40-bit encryption provides much less stronger protections than 128-bit. It is pretty easy to set key length to 40 when encrypting pdf files by using Advanced PDF Tools Command Line, the operation of which is also simple, just follow the below steps.

First, follow this link you will obtain Advanced PDF Tools

do remember to extract it, for it is a zipped package.

Second, launch the MS-DOS command prompt window

steps are as follows:

start the dialog box of “cmd.exe”: press  “start”>choose “Run”>input “cmd”>press “ok”

way to launch MS-DOS command prompt window

when the following appears, you will have the MS-DOS command prompt window

command prompt

Third, input the command line to set the key length to 40 when encrypting pdf files

it goes below

pdftools.exe -i C:\*.pdf -o D:\*.pdf  -S openpwd=123 -S keylen=0


  • -i is to specify a pdf filename or a directory for input,
  • -o is to specify the pdf file name for output,
  • -S openpwd=123 to set an open password as 123
  • -S keylen=0 refers to 40 bit RC4 encryption (Acrobat 3 or higher)

when the input is done, press “enter” key, you will see

detailed operation in  command prompt

Step 4 open the file to check if opening the file an open password is needed

window for inputting password of output pdf file

At present, you managed to set the key length to 40 when encrypting pdf files.

If you have any questions, please feel free to leave them below.

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