Hi, increasing the acidity in paper samples can be done through several methods, depending on the level of acidity required and the specific context in which you need it. If you need it for experimentation, for example, you can immerse the samples for four hours in H2SO4 at pH 3. To achieve this, mix a small amount of sulfuric acid with distilled water. A typical concentration might be around 0.1% to 1%, depending on the desired acidity level. In this case, you need pH 3. You can consult more in this publication (publication in Italian language) - ARTICOLO DEACIDIFICAZIONE Spoleto 2008.pdf (unifi.it).