Are you launching the help from the software or manually navigating from a browser to help.solidworks.com?
If you are launching from the software, do you have the option Help, Use SolidWorks Web Help turned on? If so, then there is a button in the toolbar at the top of that custom web help browser called "Zoom" that looks like two different size A's and that let's you change the font size/zoom.
If you do not have the Use SolidWorks Web Help option turned on, then you are using the local help which uses a Microsoft help viewer based on Internet Explorer and it doesn't have a UI for changing it. But, if you hold the Ctrl button on the keyboard and scroll the middle mouse wheel it changes the browser font scaling.
That Ctrl+middle mouse wheel scroll approach generally works with any web browser so whether you are accessing the local help or web help from the software (as per above) or even manually open help.solidworks.com in any web browser, Ctrl+middle mouse wheel scroll should work.
There is no way to change this default. One thing you can do is to leave the help window open while running the SOLIDWORKS session and just minimize the window instead of closing it. Then the next time you hit a help button, it will un-minimize the help window and load the new help content while leaving the font size as you left it. You will then only have to set the font size once per SOLIDWORKS session.
It has been logged as SPR# 1045682 that the font size/zoom scale that the user has set in the web help window should be remembered between help uses and even across SOLIDWORKS sessions. Please report this SPR to your reseller so they can log your name against it to give it another vote and so you will be notified when it is fixed.
Same issue here - we have large 31'' - 4k hi-res monitors and trying to use the 'help' app is anything but helpful when you have to resize the font every time. Thank you for posting this issue and for getting the problem logged.
regards - JD