What is a lightweight PDF reader, and why does it matter on Android?
A lightweight PDF reader does the reading job — open, scroll, zoom, search, copy — without the editing suite, cloud sync, ads and account of a suite. On Android the difference is measurable: install size, ads or purchases on the listing, whether you can read without signing in, and whether the app holds the Internet permission.
Why the well-known readers got heavy
A PDF suite is a reader plus an editing engine, a signing flow, OCR, conversion, cloud sync and an account to tie them together. Each is a real feature for someone. None is needed to read a document, and a reader that leaves them out can be about 4 MB.
The weight has a second cost. Ads and in-app purchases pay for the suite, and cloud sync means the file leaves the phone. For the person who only wanted to open the PDF attached to an email, every one of those is a tax on a job that should be free and private.
The four things to check on the Play listing
One: does the listing declare ads or in-app purchases? Google prints both under the app name. Two: does reading require an account, or only the premium features? Three: how large is the install?
Four — the one nobody checks — does the app request the Internet permission? An app that declares none cannot open a network connection, so it structurally cannot upload your documents, whatever its privacy policy says.
Folio PDF declares no Internet permission at all. That is checkable in the app's manifest and is the sharpest privacy fact a document reader can offer: not a promise, an absence.
Reader or suite: which do you need?
Editing, signing, OCR and conversion are suite jobs. If you read, search and copy — most people, most of the time — a lightweight reader does the whole job, and a good one opens the Word, spreadsheet and text files on your phone too.