This looks like cercospora leaf spot, a fungal disease caused by the pathogen Cercospora. It can be a problem when plants are watered overhead and the leaves get wet, or in seasons with frequent rain.

To manage the disease, always water at the base of the plant and keep leaves dry. Provide one inch of water per week to help alleviate the stress on the hydrangea. Clean up fallen leaves and dispose of them. If there are just a few leaves with spots, clip them off to keep the healthy leaves from being exposed. Clean your tools with a bleach solution to keep the fungus from spreading to other plants. Place clean mulch around the base of the plant to prevent soil from splashing onto the plant during rainfall. Keep mulch pulled back from around the plant stems so it doesn’t touch the plant stems.

Using a fungicide spray according to label directions can protect fresh leaves. This is a recommendation from Extension:

“Products containing chlorothalonil, myclobutanil or thiophanate-methyl are most effective when applied prior to or at the first sign of leaf spots. These fungicides work best to protect newly developing leaves, but they will not protect new growth that emerges after the application has been made. Consequently, to keep plants relatively free of the disease, multiple applications (usually every 10 to 14 days) may be necessary.

Remember with all pesticides, always read, and follow the label directions. Complete coverage of the leaves with the fungicide in conjunction with proper timing of applications is important for successful leaf spot management.”

More details here: Hydrangea Diseases (psu.edu)

Hydrangea: Diseases and Pests (uconn.edu)

Thank you for your question and have a great gardening summer.