It seems that your file size is too large. The maximum file size you are allowed to send is 10 megabytes. In order to successfully submit your offer, you will need to shrink the file size of your offer document to no more than 10 megabytes.
The best way to avoid this problem to begin with is to adjust your scanner (or scanner software) to scan at a lower resolution. Many scanners default to 300 dpi (dots-per-inch) or higher. That will produce a great scan, but it is overkill for scanning a document such as a purchase offer. If you can figure it out, try adjusting the scanner or software to scan at 100 dpi or so. The scan will still be very good but the file size will be much smaller; it should come in well under the 10 megabyte limit.
Another suggestion is to make sure you are not scanning in full color. If you can change your scanner or scanner software to scan in black and white or grayscale, that will also considerably diminish the file size.
If you cannot figure out how to reduce the resolution of your scanner or scan only in grayscale, your only alternative is to shrink the PDF file after it has already been created. One great product for that is Adobe Acrobat, however this software is very costly. There are however a variety of tools which are much cheaper and some are free that you can use to do this. Try using google to search for "software to shrink PDF size" and see what you come up with.
In this day and age, where sending PDF documents is so commonplace for a real estate agent, you would be well served to master the skill of creating a small size PDF document.