Soft PDF documents

Question

in fact, We have two types of PDF documents :

• Soft documents (conversion from word to PDF or Latex to PDF).
• Hard documents (conversion from scanned image to PDF).

By the way, I am only interested to soft documents.

In fact I am trying to conceal information (by using a specific steganography method...) in an existing PDF document, and I am interested to insert the embedded message by slightly modifying the position of the characters. So I know that in a line, all characters have the same y-axis but different x-axis. So I can insert some bits by modifying slightly the x-axis of each character, but if I insert bits by modifying the y-axis of characters that are places in the same line, so this will be easy detectable (because they have the same y-axis). That is why, I am interested to insert some bits by modifying the x-axis of characters which are belonging in the same line, and some bits by modifying the y-axis of characters which are belonging to different lines (each character in a specific line, but i didn't know if the gap between lines remains the same or not). And in this case, I think that my method will be more undetectable.

But before to achieve that, I am interested to get responses of the following questions:

1) If we have a PDF generated by a conversion from Microsoft word to PDF : does the gap between each line remains the same? and does the gap between paragraphs is constant (remains the same)?

2) Furthermore, If we have a PDF generated by a conversion from Latex to PDF: does the gap between each line remains the same? and does the gap between paragraphs is constant (remains the same)? Please I need your opinion and brief explanations about that.

3) When the text is justified, does the space between 2 pairs of letters remains the same ? In other word and for more precision, assume that we have a text into pdf, where the text is "happy new year and merry christmas, world is beautiful! ". The space between "ea" in year remains the same in "beautiful" ? So if we have multiple words containing "ea", does always the space between e and a is the same in all the ea of all the words? (assume that we do not change the police along all the text into the PDF).

Answer 1

You might have to explain more about what you want to do; that might make it easier to give good advice. Essentially it's important to understand the fundamental difference between applications such as Word (I'm hesitant to comment about Latex - I don't know enough about it) and PDF.

Word lives by words, sentences and paragraphs. Structured content is important and how that's layout on the page is - almost - an after thought. In fact, while recent versions of Word are much better at this, older versions of Word could produce completely different layout (including pagination) by simply selecting a different printer. Trust me, I got bitten by that very badly at one point (stupid me).

PDF lives by page representation and structure is - literally - an after thought. When a PDF file draws a paragraph, it draws individual characters or groups of characters. Sometimes in reading order, but possibly in a completely different order (depending on many factors). There is no concept of line height attributed to a character or paragraph style; the application generating the PDF simply moves the text pointer down a certain number of points and starts drawing the next characters.

So... to perhaps answer your question partially.

If you have Word documents generated by the same version of Word using the same operating system using the same font (not a font with the same name, the same font), you can generally assume that the basic text layout rules will be the same. So if you reproduce exactly the same text in both Word versions, you'll get exactly the same results.

However...

There are too many influencing parameters in Word to be absolutely certain. For example, line-height can be influenced by the actual words on a line. Having a bold word or a word in another font on a line (symbols can count!) can influence the amount of spacing between those particular lines. So while there might be overall the same distance between lines, individual lines may differ.

Also for example, word spacing is something that can quite easily be influenced with character styles and with text justification, as can inter-character spacing.

As for your question 3), apart from the fact that character spacing may change what you see, it's fair to assume that all things being equal the combination "ea" for example will always have the same distance. There are two types of fonts. 1) Those that define only character widths, which means that each combination of "ea" would logically always have the same width 2) Those that define character widths and specific kerning for specific character pairs. But because such kerning is for specific character pairs, the distance between "ea" would still always be the same.

I hope this makes sense, like I said, perhaps you need to share more about what you are trying to accomplish so that a better answer can be given...

Answer 2

@David's answer and @Jongware's comments to it already answered your explicit questions 1), 2), and 3). In essence, if you have an identical software setup (and at least in case of MS Word this may include system resources not normally considered), a source document (Word or LaTeX) is likely to produce the identical output concerning glyph positions. But small patches, maybe delivered as security updates from the manufacturer, may give rise to differences in this respect, most often minute but sometimes making lines or even pages break at different positions.

Thus, concerning your objective

to conceal information (by using a specific steganography method...) in an existing PDF document, [...] to insert the embedded message by slightly modifying the position of the characters.

Unless you want to have multiple identical software setups as part of your security concept, I would propose that you do not try to hide the information as difference between your manipulated PDF and the PDF without manipulations but instead in less significant digits (e.g. by hiding bits by making those digits odd or even, either before or after transformation with a given precision) in your manipulated documents making comparisons with "originals" unnecessary.

For more definite propositions, please provide more information, e.g.

• whom shall the information be concealed from: how knowledgeable and resourceful are they?
• how shall the information extraction be possible; by visual comparison? By some small program run-able on any computer? By a very well defined software setup?
• what post-processing steps shall be possible without destroying the hidden information; shall e.g. signing by certain software packages be possible? Such post-processors sometimes introduce minor changes, for example by parsing numbers into float variables and writing them back from those floats later.