This story, and the lovely graphic above, are from ArsTechnica, who deserve all the credit.
A lot of the charges against Manafort pertain to bank fraud. The Feds were able to establish a paper trail to use against Gates, to get him to flip, because he helped Manafort when he needed to alter a genuine profit-and-loss statement for his company, to turn a loss of more than $600K into a profit of more than $2 million, so that he could use it as part of a loan application.
Manafort’s problem was that the P&L statement was in PDF format, and he didn’t know how to alter it. So he emailed it to Gates and asked him to convert it to a Word document. Gates did so and returned it to Manafort, who then doctored it. He returned it to Gates, who then converted it back to a PDF and sent it back to Manafort.
They remind me of the comedic characters from the Sopranos.