This is the 9982nd most frequent Turkish word.
aşağıdayken
"While (he/she/it is) down below" or "when (he/she/it is) downstairs."
The word 'aşağıdayken' here refers to the state or location of the book being downstairs while the action of reading occurred.
Kitap aşağıdayken ne kadar rahatça okuyabildiğimi fark ettim.
I noticed how comfortably I could read the book when it was downstairs.
The term 'aşağıdayken' in this context denotes the time or condition of being downstairs when the thought of shopping arose.
Aşağıdayken biraz alışveriş yapmayı düşündüm ama zamanım yoktu.
When I was downstairs, I thought about doing some shopping but had no time.
Here, 'aşağıdayken' is used to indicate the circumstance of the document being located downstairs, affecting the ease of copying from it.