how to edit text size on a pdf file

Not going to happen.

Depending on who authored the PDF w/privileges, the software used and the OS it was created on, may hinder your efforts.

You can try a real PDF editor—but no guarantee there either. YOu may well be able to copy the whole of the text and create your own file with your desired text size—then export or print to— PDF dialog.

PDFpen

PDFexpert

Acrobat Reader

command P opens the print dialog>PDF>Save as a PDF

or

File>Export>PDF

Preview is not a PDF Editor, which you will need, if you want to alter any of the original PDF content. There are several PDF Editors in the Mac App Store (e.g. PDFpen, PDF Expert, etc.), or Master PDF Editor outside of the Mac App Store. All of these have free trials from the vendors' websites.