I can't be sure from your picture, but your Viburnum appears to be a Plicata tomentosum, which has a horizontal growth habit to begin with. It is also called a double file viburnum. Still, they can grow to 16' tall by 15' wide! Yours is a very attractive plant.

When selecting a shrub for your landscape there is an old saying "the right plant in the right place." Select one whose mature size will fit into your space! Pruning a viburnum as mature as yours could be a little difficult, but certainly possible. The information in the links below, although not necessarily research-based, is from  reliable sources and should be helpful to walk you through the process.

Your shrub blooms on "old wood," meaning this summer's growth will produce next year's blooms. The proper time to prune is immediately after blooming. This will give the shrub ample time to produce new growth for next year's blooms. I love the red seed heads that appear after flowering is finished, so you can decide what "immediately after flowering" means to you.

To control the height, you can safely remove up to 1/3 of the tallest stems at ground level. This will encourage new, lower growth from the base of the plant. Remove another third next year, and the final third the year after. This is called rejuvenation pruning and should give you a shorter plant. You will probably need to repeat this process every few years to keep the plant below your window. 

You should begin by removing any branches that rub against one another and any water sprouts that do not bloom. (A water sprout will grow straight up from another limb and produce no blooms.) Remove them. Then select the tallest limbs to remove this year.

https://www.finegardening.com/article/a-step-by-step-guide-to-pruning-viburnums

https://www.thespruce.com/doublefile-viburnum-shrub-2132714