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c#, check area of pdf

I need to enter a text to existing pdf (in top or bottom of the page) in c#.
I need to make sure that I dont overwrite any visible text or image.
Is there any way I could check an area in pdf if it contains text, image, control etc? I understand it will not be 100% accurate

You're going to need a full PDF consumer at the very least, because the only way to find out where the marks are on the page is to parse (and possibly render) the PDF.
There are complications which you haven't covered (possibly they have not occurred to you); what do you consider to be the area of the PDF file ? The MediaBox ? CropBox, TrimBox, ArtBox, BleedBox ? What if the PDF file contains, for example, a rectangular fill with white which covers the page ? What about a /Separation space called /White ? is that white (it generally gets rendered that way on the output) or not ? And yes, this is a widely used ink in the T-shirt printing industry amongst others.
To me the simplest solution would seem to be to use a tool which will give you the BoundingBox of marks on the page. I know the Ghostscript bbox device can do this, I imagine there are other tools which can do so. But note (for Ghostscript at least); if there are any marks in white (whatever the colour space), these are considered as marking the page and will be counted into the bbox.
The same tool ought to be able to give the size of the various Boxes in the PDF file (you'd need the pdf_info.ps program for Ghostscript to get this, currently). You can then quickly calculate which areas are unmarked.
But 'unmarked' isn't the same things as 'white'. If you want to not count areas which are painted in 'white' then the problem becomes greater. You really need to render the content and then look at each image sample in the output to see if its white or not, recording the maxima and minima of the x and y co-ordinates to determine the 'non-white' area of the page.
This is because there are complications like transfer functions, transparency blending, colour management, and image masking, any or all of which might cause an area which is marked with a non-white colour to be rendered white (a transparency SMask for example) or an area marked with white to be rendered non-white (eg a transfer function).
Your question is unclear because you haven't defined whether any of these issues are important to you, and how you want to treat them.

Not getting sharp image of rectangles on canvas

I am parsing a .sym extension font file to get the character shapes. Characters are to made using pixels and hence this font file contains information in such form that it shows which pixel is to be shown in black color and which is not be colored.
I have written logic to draw these pixels using Rectangles on a WPF canvas and everything works fine except for the fact that I am not getting a sharp image.
The purpose ultimately is to take the screenshot of this image of this canvas and paste wherever the user wants it to be pasted hence very clear image is required.
Also, when this image is exported in XPS and zoomed in, I get a very sharp image like below and hence I don't think there is any problem with the character making logic:
I don't understand why this mismatch in clarity is happening.

PDF doesn't wrap text lines automatically & respect line position

I'm trying to generate a PDF via code because not all actual PDF .NET libraries support the new Windows Runtime for Windows/Windows Phone 8.1.
The PDF is saved correctly, with only a bug of stream position count that I can fix easily, but, as you can see, the text doesn't wrap if line is too long.
I tried with PDF NewLine char (\n), but C# automatically convert it in the input string
Also, I can't understand the position of lines or objects to put into the document, because I follow this guide online that talk about a reversing axis disposition (x for height and y for width), but seems I didn't catch the right methodology (I put in my code a constant left position, at 40, and a variable top descreasing value (from 600, I'm not manage now the multipage if the value is less than 0).
This is the code of PDF generated:
http://pastebin.com/ZkZmbJdM
(Sorry if I use Pastebin, but using this editor Code function the code seems to be unformatted for special characters used for it)
Where am I doing wrong?

PDF is a graphical format trying to make you think it's a document format. But nope, it's just like drawing with GDI+ for instance. This is the reason why it can achieve the same rendered output across many platforms/programs/etc as opposed to text flow formats like doc/docx. And also, this is why it can really render anything.
So, as opposed to document formats, it is the responsibility of the program that generates the PDF to get the layout right. Think of it just as if you'd draw with GDI+.
In documents like docx or html, it's the rendering program that has to do the layout work. With such document, you just write text and the viewer will take care of laying it out.
Your PDF library certainly has the necessary code to measure the text length. Maybe even it has some code to provide some layout capabilities. You'll have to use these functions to do the layout.

How to print a table of generated images in C#

I need to print a set of generated images in a table like structure with headings. These heading and images are never displayed on the screen. The images are generate QR codes for a series of URLS so they can easier be read in by a scanner. There need to be a header on each column and a text id field on each row but apart from that the rows would be 4-6 columns of QR codes (2d barcode).
I have found samples for printing images and samples for printing text and even a sample to print a datagrid shown on the screen but nothing for printing text and images in a table like structure. Could anyone help me out with a basic example. The table is expected to go over multiple pages vertically so I would also need to put the heading row on each page.
Even pointing me in the right direction would be useful.
This is for a windows application where I want to have code kick off a print job with no user interaction. Using dotnet 4.0.

See my method for creating and printing FixedDocument here including source code. This method will easily allow you to generate aligned tables with images and text and save them as XPS or print them to a printer.

Replace text in PDF

I'm trying to replace a section of a PDF with different text. From research on all major PDF libraries for .NET, it seems this is complicated and not a trivial task. I think it may be easier to convert the PDF to an image, replace the text (always in the same place), then convert it back to a PDF (or leave it as an image if converting back isn't possible). Is it possible to extract an image from a PDF page with .NET?

If your text is in a known location, you can simply cover it with a rectangle filled with the background color, and then draw your text over top.
Note that the text will still be there, it simply won't be visible. Someone selecting text will still pick up the old stuff. If that's acceptable, it's quite trivial.

If the PDF was created from image, you can import it into Photoshop to edit it as an graphic. Or you can use screenshot program like "Snagit" to capture pdf page as image and use snagit's editor to erase old text and replace new one.
But this method may bring you problem is that the new added text may not the same font as text around it. Personally, I use pdf editor to replace text in pdf since the added text will be automatically fit with the original font and size.