Does cavity spread one teeth to another? do dentist charge to send digital x-ray to different dentist which was taken in his/her office, in same city?

Yes and some do: If an area is not cleaned with floss next to an adjacent tooth, then bacteria can increase the chance of decay on both teeth. I especially see the problem of spreading decay if the adjacent tooth has a very large untreated cavity. I do not charge to email x-rays if the patients balance is clear, to any city. Some doctors charge to send any kind of record

No/sometimes: Decay does not spread from one tooth to another. However conditions that caused decay in a tooth may also be present in an adjacent tooth. Poor home care, open contacts, food impactions, etc are all players. I do not charge routinely to send out x-rays unless it costs to duplicate them, of if the time required is excessive.

Yes and maybe: Dental decay is now considered a communicable disease based on presence of certain bacteria so it is technically spreadable but within your mouth, it is safe to say that the bacterial flora should be pretty consistent on the crowns of your teeth. A more real threat is a cavitation becoming uncleansable and increasing the danger to adjacent teeth. It is common to charge a nominal fee for xrays.