What adhesive should I use when applying Margres porcelain stoneware?

Generally speaking, you should use adhesive with mixed bonding agents (C2, according to the adhesive classifications).

See the table attached to the Margres Technical Handbook to select the most appropriate adhesive for each format and support. If in doubt, check with the manufacturer. When gluing the tiles, you should always use the double adhesive technique - adhesive on the substrate and adhesive on the tile.