Difficult to read (because of handwriting, font, layout, etc.); hard to decipher or follow.
彼の走り書きは読みづらいから、詳しく説明してもらった。
His handwriting is hard to read, so I had him explain it in detail.
この小さなフォントは高齢者には読みづらいと思う。
I think this small font is hard for elderly people to read.
縦書きの古い文献は現代の人には読みづらいことが多い。
Old vertical-written texts are often difficult for modern readers to read.