Most real tasks are a sequence rather than a single operation. Getting a stack of paper into one searchable file, or squeezing a report under a portal's upload limit without making it unreadable, takes two or three steps in the right order.
These guides are the sequences themselves, with the reasoning for each step and the decisions that change depending on what your document contains.
Scan at 300 DPI in greyscale, fix orientation and margins first, recognise once, then name files so you can find them without opening them.
Most attachment failures are one oversized scan or one uncompressed image — find the cause before compressing the whole document.
Convert everything to PDF first, standardise page size and orientation, then merge — combining in the wrong order creates most of the mess.
File size is almost always images. Find out whether your PDF is text or scan first, because compression helps enormously with one and does nothing for the other.