How to increase font size of pdf doc before taking a print out?

If you find it difficult to read the text on screen, you can zoom in to get the text readable. But, since you need to change the font for a legible printout, I am sorry to let you know that no such option exists.
You may convert your PDF file to word using the tools available online. You can change the font as you wish and take a print out easily.
One such tool can be downloaded from the link www.wondershare.com/pdf-converter/pdf-to-word-converter.html
It retains the formatting.

I did this before but I cannot recall it correctly. It can be done by any of the following methods.

You can try changing the paper type to A1 instead of A4. In printer you will be having A4 paper but as you have choose A1 it may print in bigger size.

In windows printer properties, it will show you the paper layout. There you can select only a portion of paper to print. Select the invoice portion and it will print them in magnified size.

I tried the above methods in Windows 7 OS and my printer is HP.

Other ideas:
You can zoom out invoice and take screen shot and do print out.

You can use PDF manipulation tool like Foxit to increase the font.

Some PDF Documents comes with very small fonts which may stain your eyes while reading them for long time. But you can edit that particular document with any good PDF Editor and change its font size.
I would like to suggest you to for Acrobat Pro which is famous for as PDF Editor as we all know Adobe Acrobat Reader is most used PDF Reader because of its fast performance, etc. but this time it is awarded as best pdf reader too.