How to do a reduced scan of a large document which doesnt fit into the scanner?

Reduce the scan resolution or breakup the scan job to be saved into multiple documents.
It is likely that you are using 32bits OS or 32 bits office software.

A 64bits OS + 64bits Office 2010/2013 would be able save larger documents.

Here you will find a product to compress your PDF documents http://www.systoolsgroup.com/pdf-toolbox/compress-pdf-files.html

you will find if you reduce the dpi the scanner is scanning the pictures or documents in, that you will not get a grey background , as you describe, what it is printing is actually the paper itself, if you have it set for say 1200 dpi then reduce it to 600 dpi or even 300 dpi and you will see the difference when you print it
or only select the exact area you want to scan in the document, click preview then move the scan area selection box to exactly fit the picture you want to scan

You need to install a PDF writer program first. There are quite a few freeware PDF writer programs on the Internet you can download one such as CutePDF.
Scan your document the click on File - Print and select the PDF writer as the printer device. Then name and save the PDF file.

If the document is too large for the scanner, then scan different parts of the document in two or more scans. Then open a Word document and paste the scan parts into this Word document - adjust the images to create the whole page.

Greg, Do you mean that when you want to send it by email it keeps attaching the file forever? If yes then check the size of the scanned document. What happens is that the moment you attach a document to an email Windows start sending it to the email server and that takes time as uploading is slower than downloading. And if the file a large one as scanned documents can be, then it will take that much longer. Try a smaller document first. Size wise small document. If that works fine then you can try larger documents. yes it may be time consuming but there is no price for mental satisfaction. To reduce the size of the scanned document, reduce the pixels in settings of the scanner. or reduce the page size. You will find the document reduced many times over.

I'm assuming that you mean the scanned image is to big to fit the screen, and not that the document you are scanning is too big to fit in the scanner...

Try reducing the resoloution of the scan, this will redce the size of the scanned image.

EG: at the moment your probably scanning at 1200 dpi, when 300 dpi might suit the application better.

Hi,
Check for the Save Options in scanner settings. If it is in .GIF or .TIFF format the file size obviously increases.  If you change it to .JPEG format it reduces to around 900kb.

The scanner produces a "picture" of the document, much as if you laid it flat on a table and took a picture of it with your digital camera. If you did this, your .jpg from the camera would be several hundred K at a minimum - comparable to the file size off the scanner. That's why the file is so big, it contains a picture and not text data - even though it's still a .jpg file.
If you reduce the scan resolution you will save space, but at the expense of legibility of the document image. To get it down to the smallest possible size, you will need to run the scanned image through OCR ( Optical Character Recognition ) into a Word document for example, and then print it out to a .pdf file. If you have a clean source document with few graphics this isn't as difficult as it may seem. Microsoft Office has an excellent OCR process known as Document Imaging. You need to carefully proof read the result but with a clean input Document Imaging is amazingly accurate in Office 2007.
In short, there is no easy solution to your problem.

Absent additional information like scanner, pc, size of document being scanned etc., try to reduce the resolution of the document being scanned. Large documents at high resolution require a lot of disk space, and room to cache the scan until complete. Low disk space can be an issue as well.