When you click on 'Auto-select the font size', the font size adjusts automatically. It is really helpful to choose a font that is suited to the text size that you have selected, otherwise, the size of the text will be slightly incorrect. How to Change Font Size in PDF in 3 Different Ways Sep 22, 0022 — Step 4. Press the 'Ctrl' key and the 'Alt' key together.4. In the pop-up window, click on the 'Options' button. Click on 'Settings'.5. Select the 'Change fonts' option. After this, click on the 'Change font size' button. 2. Change Font Size, size by clicking the 'Add to document' button. How to Change Font Size in PDF using Font-Size Dialog 3. Change Font Size, size by pressing the 'Add to document' button in the drop-down list.