Downloaded it. Hard to mine data as, not surprisingly, everything is in Cyrillic. The problem is that there are multiple code pages to encode Cyrillic text (CP866, KOI8-R, CP1251 at least) and the encoding varies among files. Also the most of the PDF files consist of scanned pages.

Accounting information is stored in DBF files in some well defined format. Fortunately have found a DBF to Excel converter, which sticks code page information to the output: https://github.com/rpoletaev/dbf-to-xlsx/