This isn't a duplicate of this question: I'm trying to set a bigger font size for all elements. I managed to make font size 12 instead of 10, but it's still too small to be readable.
Werner suggests in the comments to use cm-super font or anysize extension. I can's see such packages in Ubuntu. How can I install them?
Default document classes only allow certain base font sizes (10pt, 11pt or 12pt). 40pt is not supported. You need to include packages that supports that font size. Like anyfontsize or lmodern or cm-super. Then you would use something like \fontsize{40}{51}\selectfont in your document, rather than supplying 40pt as a document class option.
For installation of packages, see How to install “vanilla” TeXLive on Debian or Ubuntu? or How do I update my TeX distribution?
