If you MUST use a text field, you can read the file as a byte array, convert that to a base64 string, and store that in the text field:
string fileString = Convert.ToBase64String(memoryStream.ToArray());
or if you have an actual file on disk:
string fileString = Convert.ToBase64String(File.ReadAllBytes(@"path\to\file.pdf"));
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solved How to read a PDF into a memory stream? [closed]