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Is there a way to set page orientation to landscape when a specific element is found in OpenHtmlToPDF (or another HTML to PDF converter)?

The functionality for the project that I am currently working is to get data from a WYSIWYG editor and convert all the input to a PDF document. The problem is sometimes there is necessary to add wider tables and this produces a truncated visualization of them.
To solve this problem, I added to the editor (specifically, CKEditor) a HR button but I renamed it to "Change page orientation", so users can click that before inserting a table. In Java, I used iText 7 to detect this element (<hr>) and change the page orientation. This works like a charm.
Example using iText with a simple table
Now, requirements changed and for license purposes we need to replace iText for another HTML to PDF converter, but we need to keep this functionality.
I found OpenHTMLToPdf and I liked it, but I didn't find the way to replicate this page orientation when a hr (or another specific element) is found.
How can I solve that? I can use whatever library as long as they are open source.

Use iText to produce formatted TextFields?

Is there any way in iText to format a TextField's input? I want to have a TextField accept a phone number "(###)###-####", but I don't want the user to have to format it when they enter it. Pdf supports masks on form fields, is there any way to do this in iText?
My current solution is to create the pdf in Acrobat, then populate known fields through iText. But that isn't ideal for this deployment. Ideally I'll have iText generate the entire form.
Thanks for all assistance in advance.

You can add JavaScript to your form that changes the content of fields. See for instance the Calculator example for a fun PDF that acts as a Calculator (obviously this app only works in a PDF viewer that supports JavaScript).
When you create a text field, you need to add an additional action with the setAdditionalActions() method. You can choose between different events: K for keystroke (e.g. useful if you want to change every character to uppercase when somebody fills out a form), Bl for blurred (useful to process the content of a field as soon as the focus is lost), etc.
You can write your own document-level JavaScript to format the fields. See calculator.js for the JavaScript used in the Calculator example. Or you can use one of the many AF methods that are predefined Adobe Reader, such as AFNumber_Format (I don't find an overview of the available methods right now).

View models of type ".xml" or ".dot" files within Java JFrame

I wanted to know how to produce a JFrame that would visually show a model that is formatted in ".xml" or ".dot" format.
I know .dot format is generally used by Graphviz to view the graphs created but is there a way to use Graphviz within Java to view the models in a JFrame
So far I can create the models in .xml and .dot format and view them using Graphviz software.
My concern is just visually producing the .xml and .dot files within a java program preferably within a JFrame.

If you are open to try a new library, please check http://jgraphviz.sourceforge.net/

For some university project I once had working code to render a dot file to graphviz. The key ingredients were matching the fonts used and parsing the textual output of dot to generate Java drawing instructions based on that. So for the fully layed out output of dot this works well enough.
For XML, you'll have to be more specific about the kind of XML you have in mind, and the kind of rendering you expect. A simple document tree should be rather easy.

Insert text to a generated pdf document

I have a library which generates pdf document with images.
I want to be able to add text after each image. What is the syntax for that? How to insert text into pdf documents?
I have to use the library I have, not another one.

First of all, mkl is correct, have a look at the specification for all of the details. PDF is an exact language, if you make mistakes they will routinely be punished severely once you open the PDF in viewers.
Secondly, when you think about putting text on the page, don't forget that besides the text operators to draw the text on the page, you'll also have to specify the font to use to draw this text. Which will include making sure there is a font resource included in the PDF file if your library doesn't automatically handle all of that for you.
If you want to cut corners (I shiver while writing this) and perhaps don't read the specification as thoroughly, try this.
1) Create a PDF file that looks more or less like what you want.
2) Use a tool such as pdfToolbox from callas (http://www.callassoftware.com/callas/doku.php/en:download) or Browser from Enfocus (http://www.enfocus.com/en/products/browser). Both of these tools allow you to investigate the low-level structure of a PDF file, including looking at the actual page description code. This will show you how fonts are embedded (if you have to do it yourself that could be very handy) and how text is rendered on the page (and how you set the font, size, color etc... to use).

How to add a hidden image to a PDF document?

I have a program. It outputs to pdf, but that is close to impossible to read from again. So i need a additional file attached to my PDF in order to be able to make it editable in my program. Attaching a file to PDF is a good idea, but that is visible to the user, which i don't wan't it to be.
An alternative is to hide my readable file format inside an image which would be added to the PDF somewhere to the top of the first page, before everything else... Even to metadata if that's possible...
That way I can extract image from pdf using a PDF library (iText), and read from it.
My question is how to add image to PDF to be as well hidden as it could be (visually and by accesibility). And it has to be in a place which would be same for any created document (somewhere on the top, or on the very bottom of the document, or to the part of the document which isn't displayed at all... I'm really guessing here, I'm not really familliar with PDF file format)...
Any ideas?
P.S. It's not really important which image is it, I could be a e.g. completly transparent image, 1x1 pixels.

I'm not sure what you mean by Image, but you can "extend" the PDF reference.
A PDF consists of objects: PDF numbers, PDF names, PDF strings, PDF arrays, PDF dictionaries, PDF streams. What you probably want, is to add an entry to a dictionary (pick one: the root dictionary, the info dictionary, the root of the page tree,...) that isn't defined in the PDF reference, so that it isn't rendered in a PDF viewer.
The key of such an entry must be a PDF name. To avoid clashes with existing names (names that are part of a current PDF spec, or will be part of a future spec), it is advised to register a four-letter key with ISO. For instance, Adobe registered adbe, iText registered ITXT and use that name with an underscore. For instance, ITXT_OriginalData would be a good name if we needed the functionality you describe.
The value of such an entry will be a PDF stream. In iText, you need the PdfStream class for this.