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Topic: FDF Comments not be loaded with PDF
Conf: (P-PDF) Developers, Msg: 87476
From: svick
Date: 5/10/2003 12:49 AM

Intermittently comments will not be loaded with a pdf file despite using the proper webdav collaboration settings.

We load a template.fdf file which points to the pdf url and the fdf root folder. See sample below.

1 0 obj
/F (
/After 2 0 R
2 0 obj
Collab.defaultStore = "DAVFDF";
Collab.setStoreSettings("DAVFDF", "");
/Root 1 0 R

The collaboration settings are properly set in template fdf file. Adobe should then use the MD5 hash algorithm to determine if any comments are available. For this example the following path would contain an fdf file.

For explanation see the below excerpt from the Adobe OnlineComments.pdf document.

2.3 Data Structure
When the user opens a PDF in the browser, Acrobat reads the URL from the network layer, and then
uses Domain Name Servers (DNS) to translate the name to an IP address. The DNS conversion
guarantees that the comments will be stored in the correct repository regardless of whether or not
the users use the same URL to access the collaboration document. Acrobat then takes the converted
URL and makes an MD5 hash. For example: would be
converted to, which would hash into
UAtOJErFgHsL2xbLvPPJgC. More information on MD5 hash can be found in RFC 1321 or in many
places on the web, such as
Note: There is no way to reverse the MD5 hash to obtain the URL.
Do not change the location of the collaboration document until the period of collaboration is over.
The folder names are based on the URL of the original document. If the location of the collaboration
document is changed and more comments are added, the new comments will not be saved in
the same location as the initial comments.
When working with multiple versions of a document, name each version differently; otherwise the
comments will not align properly. In the example above, the MD5 hash for is UAtOJErFgHsL2xbLvPPJgC, while the MD5 hash for is 1N2AwJKLt6BxpzjHp4_W7B. If one was to revise
foo.pdf and place the updated document in the same location without changing the file name to
reflect the version, the comments saved under that hash would not correlate to the new document.
Highlighting may exist where there are no longer any words, or there may be line drawings correcting
a diagram that is now on another page.

Despite there being a proper FDF file in this directory, (lroyal.fdf) Adobe does not load the fdf and comments are not displayed. The fdf file does not appear to be corrupt as when the fdf file is loaded it retrieves the pdf file and the proof displays as expected.

This problem does not occur predictably and does not generate any error messages when comments are not loaded. Is there a recomended maximum limit to the number of these collaboration folders? Is a collision possible in this MD5 algorithm and if so could it cause the problem we have experienced.

Any Ideas?

-Stephen Vick

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