What a GIF to PDF conversion is really for
Before PNG took over, GIF was the format for anything with sharp edges: scanned faxes, floor plans, circuit diagrams, forms, clip art and early web charts. A lot of that material still exists in company archives and old email attachments. Pulling those images into a PDF is a way to file them alongside modern documents. The other reason people arrive here is an animated GIF they want to keep, and it is important to be clear about that case up front.
Only the first frame goes into the PDF
The converter loads each GIF into an image element and draws it onto a canvas. For an animated GIF that means the first frame and nothing else; a PDF page is a still image and this tool does not split animations into a page per frame. If the frame you want is not the first one, extract it in an image editor and convert that. For a tutorial recorded as a GIF, a series of separate screenshots turned into a PDF with PNG to PDF is the better approach.
From 256 colours to JPEG
A GIF holds at most 256 colours per frame, chosen from a palette, and a single transparent colour. The converter draws it at full size and encodes the page as JPEG at quality 92. Line art and text survive this well, but JPEG is not a natural fit for hard-edged palette images: at high zoom you may notice faint noise next to black lines that did not exist in the GIF. It does not affect legibility. Transparent regions become black, because JPEG has no alpha and the canvas fills them in when exporting; if your diagram has a transparent background, flatten it onto white in an editor first, or convert it with GIF to JPG and check the result before building the PDF.
Page size and old scanner resolutions
Each page takes the pixel dimensions of its image. Old fax and scanner GIFs are often 1728 pixels wide and quite tall, or small 640 by 480 captures. The page is sized to match either way, and the PDF viewer scales it to the window or the paper. A very small GIF will look blocky when printed on A4 because it is being enlarged; nothing in the conversion adds resolution, so if a better copy exists use that.
A worked example
Ten scanned form pages stored as 1700 by 2200 GIFs, about 60 KB each because black on white compresses extremely well in GIF, total 600 KB. After JPEG encoding each page is typically 150 to 300 KB, so the PDF comes out around 2 to 3 MB. Yes, larger than the source: JPEG at 92 spends more bytes on flat line art than a palette format does. The result is a universally readable document, which is the point. Estimates are typical, not guarantees.
Limits
The PDF contains images only, with no searchable text; for scanned forms you may want to run the pages through the OCR tool afterwards. Pages appear in the order you added the files, and the download is named images.pdf. Everything runs on your device, nothing is uploaded, and the practical limit is your device memory rather than a file count.