SumatraPDF does not attempt to reflow PDF. the main reason for being a .PDF is the page generally should be the same on ANY device or viewer, thus the line spacing should not change.
With .epub or other formats that allow reflowing of text (as the page size changes) the words should “wrap” from line to line but the underlying structure of lines such as boundaries and spacing should be maintained. SumatrPDF does allow you to change fonts and some fonts have different ratios for the distance between lines, however using a larger (or smaller) font size will not change that ratio, it will just vary the number of word per line.
The only way that most readers will show the lines spaced differently is if the source
document is set to double spaced lines etc. like this.
Thus you need a file editor to make the change.