Question: So far I've been using a printed copy of the official character sheet for my D&D characters.

It features a blank space for every stat. As a consequence of that, every time my character levels up I have to erase whatever number I wrote and write new ones. After a few levels, this wear and tear adds up and the character sheet becomes barely usable.

Sure, I could print a fresh character sheet every level and fill it up with the new stats. But, as you may have experienced, during a campaign the sheet gets covered with all kind of notes, and copying those all can be pretty boring and prone to error.

I'm about to start a new campaign, and I was searching for a custom printed character sheet that's easy to update. Possibly one with little dots or squares to represent the numbers, so that when I need to add 2 points to an ability, I can just mark two more dots. For that reason, it may take more pages than the standard sheet, that's fine.

I'd like a ready made solution, preferably a PDF that I can print.

Answer: 
You could have the character sheet printed and laminated somewhere like staples for next to nothing as far as cost and use wet erase markers to update it. This is something I have found useful during campaigns.
Use a laminated sheet and Mark it with a permanent marker. They won't accidently erase. In order to erase your old marks, write over them with a dry erase marker, then simply wipe clean.