You can use various tools to increase the font size in a PDF online for free. One way is to use Sejda PDF Editor. Upload your PDF to Sejda, click on the text you want to change and adjust the font size using the options provided.Once you've made the changes, apply them and download the updated PDF.

Another option is PDFescape, where you upload your PDF, select the "Text" tool and adjust the font size when adding new text. Similarly,you can save and download the file after making your edits.

While these tools allow basic font size adjustments, you may need desktop software for more advanced control.