Data Retained on the board

I am trying to retain the data on the board when it is transferred from my computer to the board through uart and once it is on the board through bluetooth to phone. Text document I have no issues. I am able to hold the data and then send it. But when it comes to pdf of image no data is being received. when I tried to channel the same without trying to save it first I am receiving the data properly.

I have two problem one is the same as I mentioned before. Second is that how can i increase the text size space I saw that when I am working with text 64kb is the max amount of file I can retain. As i am a beginner in this field I am kind to stuck.

I am using sdk version 16 and board is nrf52