Double click on the Fit2D shortcut and launch the program. In opened menu change X-DIMENSION to 5000 (any number slightly ≥ 4439 a number of pixels) and Y to 10000 (any number that is bigger than your number of files), so that all data can fit in. The rest should be YES, NO as shown in the figure below. To do that, click on X-DIMENSION and enter the required number below, press OK. Repeat for Y. It can crash if the input number is >10000. If you moved the fit2d window and it turns white, do "maximise" the window and then "restore down". Click "OK"

In the main menu that opens after, click "FILE SERIES" and then "INPUT 1-D":

In the opened navigation menu, find the first .chi file. To navigate use "UP DIR" and navigation/scroll buttons (most right field). A single click on a folder opens it (the right field shows files in a directory). Once you are in the folder, single click on your file. Click "YES" and then "EXIT". Select the last .chi file in the directory. Set FILE INCREMENT to "1" and press "O.K.", enter "1" to "NUMBER OF ROW TO INPUT", and press "O.K."

Go to "OPTIONS" then click "ASPECT RATIO", "NO". Click "EXIT", "EXIT". In the main menu click "IMAGE PROCESSING (GENERAL)"

The "Z-SCALING" menu allows the range of intensity values displayed to be changed and to control the manner in which data array intensity values are converted to output pixel colours. Start with a "WEAK PEAKS" option (designed to display the weak features of diffraction data), and then you can play with "+MAXIMUM", "-MAXIMUM", "+MINIMUM", "-MINIMUM" (each changes limit by 10%). "FULLY AUTOMATIC" can be convenient too. To leave this menu, click "EXIT".  A colour table can be changed in a "COLOURS" (menu "OPTIONS").
"OUTPUT" menu allows saving the 2-D image in different formats (I recommend KLORA). After choosing a format press "NO", choose the directory for an output file, input the name (do not forget to input the extension .edf at the end!), press enter.

In order to save an image in .pdf, in the image processing menu click on PRINT. Then enter a filename with .ps extension as filename.ps and press OK. It will save this image in a Postscript file on your desktop.

Then I usually convert it to a .pdf file via Adobe. It also can be opened by GSview or other software that can open Postscript files.