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What is the correct encoding to read and write PostScript files

I would like to parse a PostScript file, find appropriate line number and insert a PostScript command. So, I need to read the whole file and write it as a new file along with the new commands I want to insert.
I'm using StreamReader and StreamWriter for this process.
StreamReader sr = new StreamReader("filename.ps", System.Text.Encoding.UTF8, true);
StreamWriter sw = new StreamWriter("updatedfilename.ps",true, System.Text.Encoding.UTF8);
When doing this, even though the commands are inserted in the appropriate location, some characters are getting lost due to encoding issues.
For example, please check the below image: In the After content, you can notice the yellow highlighted characters which got added during my write process.
In summary, I would like to know the process to read and write a PS file as it is without losing data because of encoding.

File.ReadAllText does not return full content in C#

In my c# program, I have an image which is successfully stored in a byte[] data called bytes. I successfully write it into a .txt file using the following code
using (FileStream file = new FileStream("text.txt", FileMode.Create, FileAccess.Write))
{
file.Write(bytes, 0, numToWrite);
file.Close();
}
The above code stores the exact content I wish to store.
Whenever I wish to read the content of the file, text.txt, into textbox I only get the first line or little part of the first line. But when I open the file, text.txt, I see the complete content.
This is the code I use to read the file
string kk = File.ReadAllText("text.txt");

You have said at the start of the question that you have a byte[] that you are writing into the file. It's not clear why you decided not to use File.WriteAllBytes but let's assume that your code is correctly writing all the data into the file called "text.txt", which has been explained in comments does not magically make this a text file.
Using File.ReadAllText is not going to work because The data in the file is binary data, not text. As you can see from the remarks on the documentation, it will try to decide the encoding of the text file (which won't work because it contains binary data) and will do end of line processing which you won't want for a binary file.
The best way to read the data back is to use File.ReadAllBytes, which gives you back a byte[], just like you started with.

c# How to undo Encoding.UTF8.GetBytes or convert to File.ReadAllBytes

C# application was written, to transfer files to FTP server. And function below was used to read jpeg file. This is bad function because it corrupts jpeg :
StreamReader sourceStream = new StreamReader("image.jpeg");
byte[] fileContents = Encoding.UTF8.GetBytes(sourceStream.ReadToEnd());
The code below would work for the file transfer.:
fileContents = File.ReadAllBytes(sourceStream.ReadToEnd());
And now i have library of corrupted jpegs.
How to fix the mess?

You shouldn't use StreamReader at all for reading binary files, it's a TextReader. Even your 2nd piece of code is wrong, unless sourceStream only contains a file name.
It's likely that your data is corrupted beyond repair. You can do the inverse with Encoding.UTF8.GetString and StreamWriter, but your encoding has most likely caused irreparable damage already.

Replace Text in a TextFile c#

What is the best way to replace text in a text file?
I do not want to give the file a new name
I do not want the text to become one long string which is what happens when I use File.ReadAllText because this is stored as a string and I loose carriage returns etc...
Also, I guess I will run into issues using a StreamReader/StreamWriter because you cannot read and write to the same file?
Thanks

You can do it with a stream opened for both reading and writing:
FileStream fileStream = new FileStream(#"c:\myFile.txt", FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None);
var streamWriter = new StreamWriter(fileStream);
var streamReader = new StreamReader(fileStream);
...
fileStream .Close();
But the most easy way is still to read all file, edit the text and write it back to the file:
var text = File.ReadAllText(#"c:\myFile.txt");
...
File.WriteAllText(#"c:\myFile.tx", text);

Depending on your file format, you could also read your files line by line (using File.ReadLines) and perform the text replacements for each line.
You can also refer to this answer for a variant based on streams, which is the preferred way if your file is large.
How to read a large (1 GB) txt file in .NET?

Is there a restriction on the amount of text I can write / read with a stream writer / reader in C#?

I am trying to write to some text file using a stream writer.
the text I am trying to write is from a different text file.
I try:
string line = reader.ReadLine(); //reader is a streamReader I defined before
while (line != null)
{
sw.WriteLine(line); //sw is a streamWriter I defined before
line = reader.ReadLine();
}
I also tried:
while (!(reader.EndOfStream))
{
sw.WriteLine(reader.ReadLine()); //sw is a streamWriter I defined before
}
this two methods succeeded to copy the text from the file to the other file, but from some reason not all of the text was copied.
The text file I am trying to copy from is very large, about 96000 lines, and only the ~95000 first lines are copied.
Therfore, I am asking if there is a restriction on the amount of text I can write / read with a stream writer / reader in C#?
Also, I asking for some suggestions for how to succeed copy all the text.
(I read that there is a method copy of the Stream class, but that is for .NET4, so it wont help).
EDIT: I tried to replace the text in the end that didn't copied by a text form the start that was copied. I got the same problem, so it isn't a problem with the characters.

Hmm. Probably you are not flushing your stream. Try doing sw.Autoflush=true; Or, before you close sw, call sw.Flush();

I am going to guess that you are not calling flush on your output stream. This would cause the last few (sometimes a lot) of lines to not be written to the output file.