To use a custom map on a compatible Garmin Edge the map must be in a JPG format. In my example, I’m using the Sea Gull Century map offered to the public at http://seagullcentury.org/wp-content/uploads/2018/06/SGC_Route-Map-Sheet_2018.pdf
Unfortunately, like many online maps this map is in a PDF format and must be converted into a JPG. This can be done with several Adobe products such as Adobe Photoshop Elements or Adobe Acrobat Standard DC. However, if you don’t have software that can convert a PDF file, you can use GIMP a free software package at https://www.gimp.org/ or other suitable software.
In most cases, the conversion is as simple as opening the PDF file and then saving it as JPG file.