Increase PDF Size

An upload form has turned your PDF away, and not for being too big. The certificate or statement is complete and readable, but the file holds fewer bytes than the form's minimum. Here is why forms set that floor, what the tool above adds, and what happens to signatures and passwords.

Why a form sets a minimum size

Some upload portals set a minimum file size. Portals rarely say why, but a floor like this is most likely there to turn away blank or unreadable scans, which tend to make small files. A byte count is a cheap first filter, but it cannot tell a poor scan from a clean, efficient PDF. A one-page note made with our Text to PDF converter measured 3,646 bytes, about 4 KB, with every word on it. A form asking for 100 KB wants 1,02,400 bytes, about 28 times as many.

The check is on the number of bytes, and that is all this tool changes. It cannot make a faint scan clearer; if a page is hard to read, scan it again. Whether a portal accepts a file is decided by its own rules on size, type and content.

What the tool adds, and where

PDF has its own way of adding to a file after it is written: the incremental update. The tool uses it, so a sound file is never taken apart and rewritten.

A reader starts at the last startxref, reads the new index and follows /Prev back to yours, so every page, font and image is found where it always was. Nothing refers to the padding object, so nothing on screen or on paper changes. Your original bytes sit unchanged at the start of the new file.

Table after table, stream after stream

A PDF's last index is either a classic cross-reference table in plain text or, from PDF 1.5, a cross-reference stream, which is what pdf-lib writes by default. The update copies whichever your file uses. After a classic table it adds a classic table whose first entry is the head of the free list, the way Acrobat and iText write their updates. After a stream it adds an uncompressed stream, so no decoder is needed. Either way, the entries that identify the document (/Root, /Info, /ID and /Encrypt) are copied from your file byte for byte.

Landing on the exact byte

The new file is exactly the number of bytes you asked for. The arithmetic is short:

padding = target bytes − original bytes − update overhead

The wrinkle is that the padding length and the position of the new index are both written inside the file as numbers. When either gains a digit, as when the padding passes from 99,999 to 1,00,000 bytes, the file grows by an extra byte. So the tool steps the padding down until the file is at or just under the target, then fills the last byte or two with spaces inside the new trailer, after every position the index records. In the builder's tests, sizes either side of these digit changes all came out exact.

What 200 KB means in bytes

The tool counts the way Windows shows file sizes and most upload checks count:

• bytes = ⌈X × 1,024⌉ for X KB
• bytes = ⌈X × 10,48,576⌉ for X MB

A fraction of a byte rounds up, so the file is never under what you typed: 99.9 KB is 1,02,297.6 bytes, which becomes 1,02,298. The target can be up to 100 MB, which is 10,48,57,600 bytes.

These six are the tool's quick-pick sizes. A checker counting 1,000 bytes to the KB reads each of these files as slightly larger than you typed, which matters once a form sets an upper limit.

Minimums, maximums and ranges

For a minimum, type the minimum itself. A 200 KB target gives 2,04,800 bytes: exactly 200 KB to a checker counting 1,024 bytes a KB, and 204.8 KB to one counting 1,000. Either way the minimum is met.

A maximum works the other way round. Typed at the maximum, the file is over it to a checker counting 1,000 bytes a KB: 200 KB becomes 204.8 KB there, and 1 MB about 1.05 MB.

When a form gives a range, aim inside it rather than at either edge. For a rule of 100 KB to 200 KB, typing 150 KB gives 1,53,600 bytes, which reads as 150 KB one way and 153.6 KB the other, inside the range both times.

Two worked examples

A scan well under the floor. Your certificate is 1,43,872 bytes, shown as 140.5 KB, and the form wants at least 200 KB. Type 200 KB. The tool adds 2,04,800 − 1,43,872 = 60,928 bytes, all but a few hundred of them padding, and the first 1,43,872 bytes of the new file are your certificate, unchanged.

A statement a whisker short. Your statement is 2,04,650 bytes, shown as 199.9 KB, and you type 200 KB. The gap is 150 bytes, less than the smallest addition an update can make, so the tool refuses and states the nearest size it can reach. Type 201 KB instead: 2,05,824 bytes, a gap of 1,174 bytes, which leaves room.

It also shows why the counting rule matters. A checker counting 1,000 bytes a KB already reads it as 204.65 KB, above a 200 KB minimum, while one counting 1,024 reads 199.9 KB and turns it away.

When the tool refuses

The smallest addition is a few hundred bytes: 186 bytes for a simple file saved by pdf-lib with a classic table, and more when the trailer carries a document ID or encryption details.

Digitally signed and certified PDFs

A digital signature covers the original bytes, which the update leaves untouched, so the signature still verifies. Two things can still show up:

• Readers such as Adobe Acrobat may note that the file was added to after signing, which is accurate.
• A certified PDF whose author allowed no changes (a DocMDP certification with permission level P=1) may be reported as altered, even though no page changed.

The signature does break in one case. If the file's own index is damaged, an update cannot be built on it, so the tool rebuilds the file with a pdf-lib re-save first. That rewrites every byte, and the tool warns that a signature would no longer verify.

Before sending an enlarged signed PDF, open it and look at the signature panel. If the reader reports a change and the recipient relies on the signature, ask the issuer for a copy that meets the size rule instead.

Password-protected PDFs

A locked PDF can be enlarged without its password, because the update never opens the document: it copies the /Encrypt entry across and adds the padding after the end. In tests, files protected with RC4-128 and AES-256 were enlarged, and the password still opened them.

Without the password the tool cannot count pages, so its check is that the new file still asks for the same password. Nor can the padding stream be encrypted without the key. A strict validator that decrypts every object may warn about that one unused object; pypdf in non-strict mode reads past it with an "Ignoring padding error" note. Viewers never read it, because nothing points to it.

How each result is checked

Nothing is offered for download until it passes two checks. First the tool reads its output back: the last startxref must reach the new index, /Prev your file's index, and the new entry the padding object. Then pdf.js opens the new file and must find the original's page count, or the same password prompt for a protected one.

In the builder's tests, every output opened in pdf.js and pdf-lib with the same page count and a first page that rendered pixel for pixel the same as the original's, and pypdf in strict mode opened them with no warnings.

All of this happens in your browser, and the PDF is never uploaded. For a JPEG photo or signature, Increase Image Size in KB does the same job with a comment segment inside the image.

What this tool assumes

• 1 KB is 1,024 bytes and 1 MB is 1,024 KB, fractions of a byte round up, and the target is at most 100 MB.
• The input is a PDF of up to 100 MB. Memory peaks at about twice the target while building, so a 100 MB target needs about 200 MB.
• It only adds, as an incremental update that changes no page. A PDF at or above the target is refused, and so is a gap smaller than the smallest addition.
• A damaged index is rebuilt before padding, which breaks any digital signature, and the tool says so.
• A password-protected PDF is padded without the password, and the padding itself is not encrypted.
• Meeting a size rule is not the same as being accepted. Each portal decides by its own rules on size, type and content.