QUESTION: What is the difference between PDF merging and PDF stamping?

ANSWER: The terms "PDF merging" and "PDF stamping" both refer to the process of adding text or graphics to an existing PDF file. Adding text or graphics to a PDF file is done for a variety of reasons, including:

  • Putting page numbers on the pages of a document.

  • Producing variable-data documents by adding variable text or graphics; the goal here is to create a batch of documents each of which is customized or personalized in some way.

  • Adding a word such as "DEMO," "DRAFT," or "CONFIDENTIAL" to each page in a PDF file. Adding such text to a document is referred to as watermarking. The text is usually rendered in very large letters, as solid text in a very light color or as outline text, and it is often oriented diagonally on each page.

PDF watermarking

Although the terms "PDF merging" and "PDF stamping" are sometimes used interchangeably, the PDF-publishing community usually distinguishes between them as follows:

  • The term PDF merging usually refers to a process that populates the form fields on a PDF form with text and/or graphics and leaves intact the PDF code that is responsible for the "live" operation of the form field. That is: after the merging operation is complete, the form fields that were in the original PDF file still behave as form fields if you open the final PDF file in an application that allows you to interact with PDF forms, such as Adobe Acrobat. Another term for this is PDF form filling.

  • The term PDF stamping always refers to a process that adds text or graphics to an existing PDF document. However the addition of the text or graphics might or might not be accomplished by populating fields on a PDF form; there are other ways to add text and graphics to a PDF file.

    The key thing to know about PDF stamping vs. PDF merging is that when you're doing PDF stamping, if the text/graphics were incorporated into the PDF file by populating form fields, the form-field code is removed from the PDF file during the stamping operation, leaving behind only the text or graphic that was inserted into the form field. That is: if you open the final PDF file in an application such as Adobe Acrobat that lets you interact with PDF form fields, you will not be able to interact with the areas of the document where form fields exist in the original PDF form (the one that was used as a template during the stamping process). In other words, after the stamping process is finished, the text and graphics that were added to the original document are just like any other "regular" text and graphics in the final document.

    Form flattening is the term that's used to refer to the process of removing form-field code from a PDF file after a form field has been populated with text or graphics.

Depending on the goal and desired result of the merging or stamping process, the original PDF file is either altered (that is, the new text/graphics are added to it, and it is saved under the same filename) or, conversely, the original PDF file remains intact, and a new version of the PDF file is created (it contains the text and graphics from the original file plus the text and graphics that were added during the merging or stamping process, and it is saved under a different filename).

Why is form flattening important?

The issue is that if you are creating new PDF files by using software to add text or graphics to existing PDF files, your final documents could be altered quite easily by someone that is using software that is readily available in many environments (for example, Adobe Acrobat) if the final documents contain form fields.* That is a good thing if you are creating PDF documents that are supposed to be altered after they are created — but if you are creating PDF documents that not supposed to be altered, it is not good if they contain form fields. Here are examples of both kinds of situations.

  • Let's say that you're developing an automated system that generates mortgage-application forms in PDF format by using software that populates a PDF form with information from a database (name, address, phone number, etc.). The goal is to make it easy for potential mortgage customers to apply for a mortgage by filling in most of the information on the form and then making the form available to the potential customers in electronic format so they can use software such as Adobe Acrobat to read the information on it, change any information that's incorrect (e.g., perhaps they have a new phone number), and add information that is missing.

    In this situation, it makes sense to do PDF merging, not PDF stamping, because you want to have form fields in the PDF file that you create so that potential customers can fill in the required information.

  • Now let's say you work for a company that publishes college textbooks, and the company maintains its electronic inventory of textbooks in PDF format. You are developing an automated system that can generate a new version of an existing textbook by putting a watermark on each page. The watermark says "EVALUATION," and the idea is that college professors can obtain from your company a PDF file that contains a watermarked version of a textbook for the purpose of evaluating the contents of the textbook to decide whether or not to buy many copies of that textbook for an upcoming semester. You apply the watermark to the PDF file because you want to discourage dishonest college administrators or dishonest professors from using the PDF file to print their own textbooks without paying anything to your company.

    In this situation, you do not want to make it easy to remove the watermark from the pages of the textbook — and it will be very easy to remove the watermark if the watermark is contained in a form field. It is a simple matter of opening the PDF file in Adobe Acrobat, clicking on the form field that contains the word "EVALUATION," and deleting the text.

    So you'd want to do PDF stamping, not PDF merging, in this case.

How do you do PDF merging and stamping?

If you are a programmer, you could obtain the Adobe PDF Reference, read it, and write your own software for doing PDF merging or PDF stamping — but if you did that, you'd be doing it the hard way! There are quite a few commercial software products that allow you to do PDF merging and PDF stamping. Our company offers three such products: Quite Imposing Plus, Quite Hot Imposing, and XpdfSplice.

*Even if the final PDF documents don't contain form fields, it is still possible to alter them in most cases (though altering them is easier if form fields are present). If you need to make sure that nobody alters the PDF documents that you are creating, you should encrypt them. Encrypting them involves assigning passwords and specifying security parameters.

If you are creating only a few documents, you can simply use the Adobe Acrobat software on your desktop to encrypt them, but if you doing merging or stamping of large numbers of PDF documents dynamically, you'll need software that can encrypt them on the fly. You can us the XpdfSplice software to do this.