I have re-read the parameters of this project several times. If your brother-in-law just wants the data, not a restoration, get a polarizing filter to handle glare, get as close to the document as you can without cutting off data, take 3 or 4 overlapping pictures, PP as necessary and send him the JPG,s.
I do this kind thing all the time in that I do lots of restoration and documentation work for archives and museums.
Do not attempt to remove the document- it may stick to the glass and becomes severely damaged. Just clean the glass very gently with a dampened cloth and if there is any stubborn dirt, use a mil mixture of vinegar and water- NO WINDEX!
The BEST way to copy your document is with cross-polarized light, that is, 2 lights, each a 45 degrees to the document with polarizing filters ON THE LIGHTS as well as a CPL filter on the camera. There will be no unwanted reflection and you will improve the contrast. You can use flash, incandescent or LED light sources. The polarized screens on the lights must be oriented in the same direction. Rosco makes polarized flexible plastic sheeting. There are othere manufacturers as well.
You need strobes with modeling lamps or continuous light sources so you can see the effect as you rotate the filter.
If you don't want to go through all this procedure or buy the polarized sheeting, you may be able to save the last shot you downloaded or make another more carefully focused and centered shot withou the keystoning and post process it as I did in the attached quick edit: I corrected the perspective, sharpened the image, desaturated the color so as to eliminate the stains, bumped up the contrast, restored a bit of warmth to compensate for the desaturated tone, and cropped.
I hope this helps
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