Let's suppose i have a hexadecimal array with 16 cells.for example
b3e2d5f636111780

i want to convert it to an array of ascii characters(in C) so that
i can reduce total size of the file i want to put it in.
But i am afraid i have not fully understand the difference between ascii
and hex(i thought that an ascii array is simply a char* array but
this seems wrong).
Thank you in advance.

Ascii characters are made up from one 8 bit byte. So a "|" (pipe symbol) is represented by a value of 179 decimal, or b3 hexadecimal.
What you have in your original string is 16 8-bit bytes. If you take these in pairs and multiply the first by 16 and add the second, you will end up with a value between 0 and 255, (b *16 +3 = 11 * 16 +3 = 179), which you can then store in a single 8 bit byte, resulting in a 50% reduction in storage.

thank you for your answer! so you suggest that i do that in a for(){} condition and that i write every byte that in each repetition occurs into the file?

There are a few more things to consider. First, strings with an odd number of characters must be flagged somehow so that the resulting representation can be converted back into an odd number of characters. Second, you're turning a string into the native representation of an integer. So, you need to decide which endianness the storage will have if you want the resulting file to be compatible across architectures.