Change font size in a pdf file without changing plot size

I am trying to include a pdf file in my work, where I plotted a graph. In the pdf there is a graph and axis labels. When I am resizing the pdf to fit in my A5 latex pdf, the axis labels are getting very small and unreadable.

begin{figure}[htbp]
    centering
    includegraphics[scale=0.4]{PDFs/Training.pdf} 
    caption{Training}
end{figure}

I am able to highlight the text in the pdf, so I am thinking it is possible to change the font size of the pdf with maintaining a smaller scale to fit the A5 page.

I looked to some old questions here, but non is actually solving this problem.

Thanks in Advanced!