Hard to read; describing text or handwriting that is difficult to decipher or understand (due to font, layout, handwriting, wording, etc.).
このメールのフォントは小さすぎて読みにくい。
The font in this email is too small and hard to read.
手書きのメモが乱雑で読みにくかったので、確認の電話をした。
The handwritten note was messy and hard to read, so I called to confirm.
古い書類は色あせていて読みにくい部分が多い。
Many parts of the old document are faded and hard to read.