On some PCs when a PDF of an invoice is generated, the fonts do not always appear as they look on a print preview of the invoice. Sometimes a slightly different looking font is used and sometimes text looks slightly larger than in the preview.
To make the PDF look exactly like the print preview, check in Windows to make sure that default (100%) size fonts are selected. with new installations of Windows 7 for example, even when upgrading from an older version of Windows on the same PC, Windows will choose to set medium-sized fonts without asking.
On Windows 7, to check text/ font size and to switch to the default (100%) setting, see this Microsoft support article:
On Windows 8, go to the Start screen and search for "Make text and other items larger or smaller".
Then choose the text size as Default.
After changing the font/ text size, try generating a PDF invoice again to see if it now matches the print preview. If you find that text on your PC screen is too small, try adjusting the screen resolution to make items appear larger.