Publishers have traditionally used 2 column layout for printed articles. These have then been turned into pdfs.
Okay, nowadays, some things only appear electronically. However, you, me and most of the rest of the world find them hard to read even if using a large computer screen. Reading them on things like phones is difficult.
Therefore it makes sense to lay out your work in the most readable form. Why you would publish it in two columns is beyond me. Make it easy for the reader and then they are more likely to read, and cite, your work.
In answer to a comment: there are various pdf editors. However, I haven't found one that will accurately edit a pdf it didn't create unless the layout is very simple e.g. no header or footer, no page number, no title, only one column, etc. I usually find it is quicker to turn them into text and re-format, or just re-type the whole thing (something I do quite regularly).