When a computer reads a PDF...
You can take several perspectives.
From Acrobat reader, it asks the operating system to read a file, copies it to memory then interprets drawing commands. From applications POV, caches don't exist.
Of course, there is nowadays a disk cache in RAM, where the last used sectors are kept for shorter access time. Some OS even trick the MMU between the disk cache and application address spaces so that data is not even copied. That cache is not visible to applications (which can also have their own caching, for example pre-rendered pages...)
From the operating system, CPU caches needs sometimes to be manually managed, on some platform, particularly when there are peripherals that are modifying memory without CPU intervention ("non coherent DMA"). It could be the disk controller reading that PDF file. On current high performance CPUs, the kind you find in computers or tablets where Acrobat can be installed, the CPUs practically manage their caches transparently.
So, CPU caches are mostly hidden to software.
Cache are made of two parts : the datas, and the tags.
Datas contains a portion of RAM area.
Tags indicate which part of the RAM is in the cache.
Data is grouped in chunks ("cache line"), usually 32 to 128 consecutive bytes.
When a CPU accesses its caches, it lasts usually less than 5 cycles for L1 caches, and less than 20 cycles for L2 caches.
Accesssing DRAM can waste hundreds of cycles.
Despite being very small (for example 32kB L1 compared to 4GB DRAM), they are incredibly efficient, because, in reality, accesses are often grouped, or consecutive, or repetititve, so that the same parts of main memory are accesses several times.