2) With your method of representing the digits, how will you, for example, store the 30000th digit only in 1 byte into a file? I need any digit to be represented as 1 byte exactly.

let me demonstrate the problem with base 65535, (or any base greater than 255):

1st digit = chr(1)
2nd digit = chr(2)
3rd digit = chr(3)
.
.
.
255th digit = chr(255)
256th digit = ????? ' no more available ASCII codes.

Any idea?
Yep, youre still trying to put it in a string instead of a integer array. Now there's the integer that is just perfect for this issue since its ranged from -32,768 to 32,767 that means you enter the value into the integer as follows:
Code:
IntArray(decimalplace)=digit-32768
where digit can be ranged from 0 to 65535

To store it in a file, open it binary:
Code:
Open file for binary as #1
Put#1,,IntArray
Close #1
So, it's actually very easy to get around this prob.