You can create one Rule-Obj-HTML rule and define the styles in it between <style></style> tags. On any HTML rule used to generate PDF's first include that HTML file.
You can also add inline style to a smart layout section. Include the section into a HTML. Use the Property-Set-HTML or Property-Set-Stream method and pass the above HTML rule. Use “HTMLToPDF” OOTB activity that forms the pdf data.
Pega has provided one css file for pdf "pyCompactStylesForPDF" which has minimal styles ,you have to include that in your HTML stream where you are including the section in HTML Stream. Then it picks most of the styles but we need to make changes according to our requirement which you have to do by overriding the css.